David Bradley is no stranger to Doctor Who. In 2012 episode Dinosaurs on a Spaceship he played ruthless merchant Solomon and a year later, for the show’s 50th anniversary celebrations, he starred as William Hartnell, the First Doctor, in An Adventure in Space and Time, Mark Gatiss’s drama about the genesis of the show.

But now Bradley can legitimately “join the ranks of the legends”, having appeared as the actual Doctor in actual Doctor Who, popping up at the end of the series ten finale and set to appear alongside Twelfth Doctor Peter Capaldi in his last ever adventure this Christmas.

“It’s something that I never imagined when I did my Dinosaurs on a Spaceship about five years ago,” said Bradley, speaking at London Film & Comic Con at the end of July. “I thought ‘wow, that’s the Tardis, wow, that’s the police box, well this is my Doctor Who experience and that’s gonna be it but that will be fine’. And then eventually Space and Time came up, but after that I still couldn’t call myself the Doctor – I was William Hartnell and his version of the Doctor – but I was told that [after the Christmas special] I can now join the ranks of the legends.”

But could another actor also be joining those legends in the Christmas special? We know Capaldi’s Twelfth Doctor will regenerate in the episode but based on what we’ve seen in the trailer, so could Bradley’s.

Although unconfirmed, everything points to the initial setting for the episode being the South Pole in 1986, the scene of Hartnell’s last outing, The Tenth Planet. The First Doctor’s speech at the start of the trailer is from that story, the blizzard conditions fit, it features companion Polly, and the title, Twice Upon a Time, sounds very much as if it could hint at a return to a moment the Doctor has lived through before. Add to that the fact that the trailer shows that telltale golden glow emanating from the First Doctor’s hands and it looks as if we’re in for the show’s first double regeneration.

Of course, that glimpse could be as much as we’ll ever see – perhaps we leave the First Doctor at that point. But if so, why blow it in the trailer?

And if we do see it through to its conclusion, that means a new (or perhaps old) face for the Doctor.

So who could it be?

Gatiss’s League of Gentlemen co-star Reece Shearsmith played Second Doctor Patrick Troughton in a brief appearance in Space and Time, so does that mean he’ll follow Bradley into Who canon? I don’t think so, and I hope not.

Bradley earned his place on Doctor Who partly because he looks a lot like Hartnell did at that age and partly thanks to an uncanny performance as the Doctor in An Adventure in Space and Time (during the Comic Con session he was asked to read the First Doctor’s farewell speech again and was so bang on that it sent shivers down my spine).

I’m not criticising Shearsmith’s performance (there wasn’t enough to criticise) but he looks nothing like Troughton. He lacks the hawklike features, and his relatively slight frame in an oversized jacket, along with that unconvincing mop-top wig, combined to make him look more like a clown than a Doctor.

Beyond that, though, transferring one actor from a drama about Doctor Who to the show itself is just about acceptable, but doing it with two feels like smashing the fourth wall to bits and I have a feeling that’s a sentiment departing showrunner Steven Moffat might share.

So if we do see the Second Doctor, I don’t think it will be Shearsmith.

On one hand, the prospect of having a new actor play Troughton’s Doctor is quite exciting, even if it would set a record for the shortest stint ever as the Time Lord (unless series eleven is going to take a very unusual turn and follow the adventures of both the Thirteenth and Second Doctors – ha! I wonder if that would placate the fans who are still struggling to accept a woman in the role).

Patrick Troughton’s actor son Michael would no doubt be happy to pay tribute to his dad in a one-off appearance, and he certainly has Patrick’s nose and mouth, even if he is fuller-faced. Could he step in?

More likely, it seems to me, we’ll glimpse the real Troughton’s face surrounded by regeneration energy as Bradley changes – much as we did when John Hurt’s War Doctor became Christopher Eccleston’s Ninth.

That’s easy enough to do with CGI these days – Eccleston wasn’t involved in The Day of the Doctor, they just used old footage. Hell, maybe the BBC will even up the budget for Moffat and Capaldi’s last episode (after all, it is Christmas) and give us a few moments of Troughton in action, post-regeneration.

When I asked David Bradley about all of this, he was understandably guarded. “As far as I know you don’t see anything like that,” was his initial reaction, but he quickly followed it up by suggesting there could be something to hide, even if it’s not Reece Shearsmith. “I mean, I don’t know what’s a spoiler,” he said. “Even if I knew I couldn’t really say. But I never saw Reece.”

Of course, two Doctors don’t need to be in the same room in order to shoot a regeneration scene – by all accounts, Peter Capaldi filmed his departure long before Jodie Whittaker shot her arrival (if she’s even done it yet) – so there’s no reason that Bradley would have seen Shearsmith anyway. But I want to believe he’s not involved so that’s what I’m going to read into Bradley’s words.

As for how much of the First Doctor’s regeneration we’ll get to see, well it would seem strange if an adventure that looks set to recreate scenes from Hartnell’s final episode (which currently remains lost) did not include the most significant moment in Doctor Who history – the introduction of the very idea, regeneration, that means the show is still going half a century later.

There would be a pleasing symmetry to it, too. Two Doctors, the first and the latest, two regenerations, two departures – for Capaldi and Bradley – and a neat signing off for Steven Moffat, creating yet another of the big moments that have become a mark of his time in charge of Doctor Who, in an episode that increasingly looks as if it is going to be chock full of them.