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!

Are you a tough cookie in real life?

Mentally, I think I am. Which is probably why I can act a good fight and throw a good punch. Physically, I’m a delicate flower.

Did you have to get a head cast for your alien mask?

Yes, but to be honest I fell asleep while they were doing it. The cast goes over everything, so it’s really warm and you can’t hear anything. It was nice to just nod off.

How do you find the juggle between acting and being a mum?

I feel lucky to have a balance between the two. I have a great family who help me out.

Class is filmed in the same studios as Doctor Who. Did you manage to sneak a trip on the TARDIS?

I did! Orla was running round and pressing all the buttons. We loved it.

Class continues on BBC Three; episode three is available from today.