She is now starring in the new Doctor Who spin-off Class, where she plays a fly-kicking physics teacher (and undercover alien) called Miss Quill. Katherine, 36, lives in London with her Australian husband Ryan Clark and their two-year-old daughter, Orla.

Your new role in Class is quirkily different. What attracted you to it?

Exactly that: it was so different. I like to change things constantly. Variety is the spice of life for me and I just thought this show was brilliant. I loved its unpredictability.

Have you done sci-fi before?

No – though my brothers were mad on it as kids. We were a bit young for the original Doctor Who, but Star Trek was a big thing in our house.

What I like about Class, though, is that it’s very rooted in reality. It’s set in a school – Coal Hill School from Doctor Who – and the main student characters are totally believable.

How would you describe Miss Quill?

She’s the most un-PC teacher in existence. She thinks her students are incredibly thick – and she tells them they are, too. She doesn’t really like anybody. But what most of them don’t know is that she is also an alien in human form!

Does the part involve much action?

I think I have a fight in pretty much every episode. Some of it was done by a body double, but I had to do quite a bit of it, too. It turns out that fly kicking in high heels isn’t the easiest thing to do!

After a rather frenetic and breathless start, Class eases the pace and is richer and more affecting as a result – ‘Nightvisting’ is the Doctor Who spin-off’s third offering and it’s easily its strongest yet.

The episode takes its title from folk singer Jim Moray’s 2006 track, which also scores a powerful opening montage charting the marriage of Tanya’s parents – from its sweet beginnings to its bitter end, with her father’s body removed from the family home after he suffers a fatal stroke.

Two years later and Jasper (Kobna Holdbrook-Smith) reappears in his daughter’s bedroom, having “reached out to [her] across all time and space” – it’s but one of many apparent resurrections occuring across London, but are they genuine or a sinister deception?

Ram (Fady Elsayed) is, of course, haunted by a vision of his dead girlfriend Rachel (Anna Shaffer) – and in exploring his prolonged guilt over her death, Class actually gets one up on the hallowed Buffy, which offed a character beloved of the Scooby Gang in its pilot… then never mentioned that character again.

Instead, Ram here has to face Rachel’s loss all over again, but finds some unexpected solace as he and April (Sophie Hopkins) – two of the few left unaffected by the alien threat – are thrown together.

April was already friends with Tanya and (to a lesser extent) Charlie before the series began, while Ram had formed a secret connection to Tanya and even he and Charlie shared a relationship of sorts.

Ram and April, then, seem like the unlikeliest of couplings – even before you consider their polar opposite personalities. He’s like a raw nerve, wearing his heartbreak on his sleeve, while she’s thoroughly repressed as the only way to get by.

But when April opens up to Ram about her alcoholic, wayward father, it brings the pair closer together – a lot closer – and it’s impressive how Patrick Ness is able to build a credible and appealing romantic connection between the two in less than 25 minutes.

The premise of ‘Nightvisiting’ also allows Katherine Kelly the opportunity to dig a little deeper as Miss Quill, a character that Class mostly used as comic relief in its opening double-bill.

While it’d just be plain wrong for Quill to start pouring her heart out, we do get to glimpse a few cracks (no pun intended) in her apparently impenetrable frosty exterior – and without ever compromising the character.

Charlie (Greg Austin) meanwhile is haunted only by the briefest of visions of his parents: “Our bond was not strong,” he explains. “I’ve got different ones that are stronger.”

His emotional development comes not from a heartrending exchange with a lost loved one, but from a heartfelt one with Matteusz (Jordan Renzo) – the pair swapping declarations of love and even moving in together after Matteusz’s homophobic family kick him out onto the street.

But for all the flurry of confusing and conflicting emotions flying through the air – ah, teenage life – it’s Tanya’s pain that proves most powerful and devastating of all.

Vivia Oparah has to shoulder some difficult material as Tanya debates whether or not to ‘pass over’ and the actress doesn’t disappoint. But in the episode’s final moments, rather than wallow in grief, Tanya instead revels in her anger and how it’s made her stronger – a fitting and strangely uplifitng note on which to end.

‘Nightvisiting’ is light on action – boiled down to its basics, it’s a series of conversations in a series of rooms. But with Ness tackling tough subject matter with confidence and his cast responding in kind with emotive performances, the end result is supremely watchable.

Those early growing pains appear to be done with, because this week Class delivers a powerful, witty and poigant piece of drama – one we’d be happy to revisit.

Over in the States, there has been a lot of hubbub about Hillary Clinton’s deleted emails. Now, one purported email, signifying her disdain for Doctor Who, may truly be the end of her campaign.

As the 2016 Presidential Election here in the United States is coming down to its final two weeks, the attacks and rhetoric are reaching a fever pitch. One of those hot topics, since almost the very beginning of the election cycle, has been that of Hillary Clinton’s deleted emails. Wikileaks has released quite a few of them, with each batch promising to be the end of her campaign.

However, there is one email* that has come out that may truly signal the end of her hopes for the Presidency. Instead of being political in nature, or housing secret government information, this email discusses Doctor Who — or, rather, how she just does not appreciate the show, calling it “boring garbage” and feeling as though she is being left out on a joke that everyone else understands.

Can't believe the latest #Wikileaks release. Hillary is DONE! pic.twitter.com/4YnGZIUjbn

— Mike Drucker (@MikeDrucker) October 24, 2016

She even questions if the quality of Doctor Who is a signifier that “England is out of money,” which is quite an insult toward an ally of the United States. Could this cause an international incident?

Now, it is understandable that Doctor Who may not be for everyone. After all, watching a Time Lord who is several thousand years old zipping around time and space with whatever British female he picks up can be a bit much for some. Add in the various plot holes, or the stories that are left unfinished, and it can get quite confusing.

So, maybe that explains why Clinton is not a fan of the program. She’s been busy over the past few decades, helping her husband’s political career before embarking on her own. She just may not have time to sit down and really watch the show with the same level of attention that others provide, as her mind is likely elsewhere.

But to say that Doctor Who is boring garbage? Well, that crosses a line that few would dare to verbalize. In saying that, she has, in effect, removed the Whovian demographic from her voting population. Yes, she has a somewhat higher opinion of Sherlock, which has a great deal of overlap in terms of fandom, but to attack the Doctor? That’s a bold move, Cotton.

Fifty years ago tomorrow, at 6:13pm, a unique moment in television history took place. A camera closed in on the face of an actor lying motionless on the floor. The image blurred, faded to white, then came back into focus, revealing that for the very first time, Doctor Who had changed his face.

It was the stroke of genius that saved the series from being a three or four season wonder, and instead ensured it would go on to become one of the UK’s most iconic pieces of popular culture.

Switching the lead from William Hartnell to Patrick Troughton was an audacious move, brought about because of falling ratings, plus Hartnell’s ill-health and attitude.

And whereas the announcement of a new Doctor in the 21st-century merits a live TV unveiling with accompanying liveblog, in 1966 the Manchester Guardian noted that the childrens’ television show was changing its lead in the same news-in-brief column that announced who had won Miss Blackpool.

Troughton arguably had the hardest job of any actor in the role – namely to convince the public he was the same person they had been following since 1963. In his first episode, the brief dialogue trying to explain what had happened was aimed as much at the audience as at his companions, Ben and Polly, in the TARDIS.

Interviewed in 1986 for Fantasy Empire by John Peel, Troughton said: “I didn’t think it was a particularly good idea of the BBC to replace Billy. I thought it was pretty silly, really. I thought that the difficulties of selling it to the audience were enormous … almost insurmountable. However, in the end, I was persuaded over a week of negotiations, and I thought, ‘What the heck, let’s do this for a while and see what happens’.”

Troughton’s first story as the Doctor – The Power of the Daleks – is lost, but will soon be issued by the BBC in animated format. He went on to play the Doctor until 1969, then reprise the role during the 80s in The Five Doctors alongside Jon Pertwee and Peter Davison, and The Two Doctors with Colin Baker.

When Hartnell turned into Troughton, the show had very little back-story. Doctor Who was not even yet a Time Lord from the planet Gallifrey; his origins were still a mystery. Even when Hartnell was shown facing one of his own people for the first time (the Meddling Monk), details of their home world were entirely absent. One can only imagine how surprising the reveal was in 1965 that somebody else even had a TARDIS.

We now exist in an era where Spider-Man can be effectively rebooted three times at the movies in the space of 15 years, but when Doctor Who did this, it was very much uncharted territory. Up to this point, most changes of actor had either been simply ignored on-screen, or been done by hastily bringing in new characters to cover for an absence.

And few other series have made such a radical recasting of the lead an intrinsic part of its formula.

Take Star Trek, a similarly long-running fantasy franchise. After its original run, both the 70s animated series and the initial batch of motion pictures relied on the same actors and characters. It wasn’t until over 20 years after the first episode aired that they dared replace William Shatner with a new captain of the Enterprise.

No such luck for Hartnell, who got three years as Doctor Who – and remains the only actor to take the role who didn’t realise that he would one day be replaced.

As well as “the renewal” (the term regeneration was not used at the time) Hartnell’s final story also introduced another enduring part of the Doctor Who universe: the Cybermen. The creatures depicted in The Tenth Planet are more obviously human than later designs (and DVD remastering shows how much the monsters from Mondas relied on Sellotape to stop them from falling apart).

Still, the costume designs are chilling. The stretched cloth across the face, and the hollow eyes, give a clearer indication of surgically-enhanced humans than the later faceplates. But this was actually a cost-cutting exercise: their faces were hidden so the same handful of extras could play different groups of the space invaders across all of the episodes.

Hartnell was too ill to shoot part three of the story, the week before he was due to leave the show – so it was hastily rewritten to install an incapacitated body double in a bunk bed.

And, since the last episode is missing from the BBC’s archives, there are only fleeting glimpses of his Doctor’s end. It’s hard to judge whether his erratic performance in those final scenes are as intended by the director, or a product of his health.

There is a recently discovered clip of him being interviewed while getting ready to do a pantomime performance three months after his departure. Asked whether he would “shake off” being Doctor Who, Hartnell was bullish: “Oh yes. By making a success in something else, why not?”

But it wasn’t to be.

Hartnell’s career essentially ended with that regeneration scene. He featured briefly in early 70s story The Three Doctors, and there’s also a wonderfully bizarre cameo from him, which has turned up on YouTube, in a Cliff Richard musical from the late 60s which retells the parable of the prodigal son. (Hartnell improbably plays a shop owner who is Sir Cliff’s father.)

Hartnell never did shake off playing the Doctor. But, as Doctor Who heads into its sixth decade, it’s clear that the decision to shake him off meant everything.

The essential premise of Doctor Who – a traveller in a blue box who can go anywhere in time and space, putting wrongs to right with the power of intellect rather than force – now falls in the same category as Sherlock Holmes, James Bond, Miss Marple or Robin Hood: characters that we know well, and that have an endless capacity for renewal. People continued to want to tell tales of the Time Lord when the show was largely off-air between 1989 and 2005, whether as comics, novels, audio dramas or webcasts. None of that would have happened without that gamble back in 1966.