There’s one element of Doctor Who that I haven’t mentioned yet: regeneration. In a way, it’s the most important, because without it, the show wouldn’t have lasted for five years, let alone fifty. If the lead actor becomes too ill, or too difficult, or too unpopular, he can simply be replaced.

It was Doctor Who’s then-producer, Innes Lloyd, who came up with this idea after the First Doctor, William Hartnell, had become increasingly erratic in his ability to play the part. Because the Doctor was an alien being, he thought, there was nothing stopping him from renewing himself into a younger, healthier person. The character of the Doctor could be defined by the person who played him. Suddenly, a programme that had a finite shelf life could now last for ever, continually reinventing both the lead character and the format of the show.

I learned that other Doctors existed on Saturday 8 June 1974, the day the Third Doctor became the Fourth. An hour before the episode was broadcast, my mother sat me down and told me, in no uncertain terms, that my childhood hero was going to die.

Mum didn’t use the word regeneration. No, this would be a resurrection. Just like Jesus, she said. Or the time our goldfish threw itself out of its bowl, or when next-door’s cat was run over by a milk float. And in much the same way that the next-door neighbours got themselves a new cat, and we might get another goldfish one day (we never did), today everybody would get a new Doctor Who.

The regeneration itself lasts all of five seconds. It’s a simple visual effect – as simple as it gets. The camera is locked-off, the actors lie down in the same spot and then they mix the two images together. But it’s still one of the best tricks Doctor Who ever managed to pull off.

To date, the Doctor has regenerated eleven times. I imagine you could name the twelve actors who have played the part on television, but just in case you can’t, they are: William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, John Hurt, Christopher Eccleston, David Tennant, and Matt Smith.*

However, it could be argued that many more actors have taken on the role of the Doctor, even if the part they are playing is incorrectly referred to as Doctor Who. These Doctors exist in a non-canonical alternative Whoniverse of comedy sketches, feature films and stage plays outside of the main Whoniverse (don’t bother looking in the dictionary, it’s not there).

Discussions of what is and what isn’t part of the Doctor Who ‘canon’ are to the programme’s fans what angels dancing on the head of a pin were to medieval theologians. Friendships can be wrecked over whether, say, Tom Baker’s appearance in his Doctor Who costume on Disney Time in 1975 means that it was actually the Doctor who materialised on Bank Holiday Monday to present Disney Time during the four-month break between ‘Revenge of the Cybermen’ and ‘Terror of the Zygons’ (for the record – of course he bloody didn’t).

However, should you ever find yourself ensnared in a tricky pub-quiz situation, and you are asked to name more than twelve actors who have played the Doctor, here are some of the less well-known regenerates:

Trevor Martin. Trevor Martin is a fabulous actor with an impressive CV of film and stage roles. However, in 1974 he played Doctor Who in a West End stage production entitled Doctor Who and the Daleks in the Seven Keys to Doomsday. For one small segment of the population, this means the actor’s numerous appearances in plays by William Shakespeare and the like pale into insignificance compared to the three weeks he spent hanging around the stage of the Adelphi Theatre, waiting for a Dalek to trundle onto its mark.

Richard Hurndall. For the twentieth-anniversary story, ‘The Five Doctors’, neither Tom Baker nor William Hartnell was available to take part, one because he was sulking and the other because he was dead. Hurndall stepped in, giving an uncanny impersonation of the irascible First Doctor, except for the fact that he managed to deliver all his lines without mucking them up.

David Banks. More usually concealed inside a Cyberman costume on television, David Banks was also Jon Pertwee’s understudy for a 1989 musical stage play called Doctor Who: The Ultimate Adventure. When Pertwee fell ill one day, Banks grasped the nettle with both hands, and for the lucky people who saw that matinee performance, it is said, not least by David Banks, that he gave the definitive portrayal of the Doctor. Unfortunately, the number of people who claim to have seen David’s Doctor far exceeds the seating capacity of Birmingham’s Alexandra Theatre, so this hyperbole should be taken with a pinch of salt.

Richard E. Grant. Two for the price of one, here. Grant has played the part twice, once in 2004 in a computer-animated story called ‘Scream of the Shalka’, and previously in 1999 when he appeared alongside Rowan Atkinson, Jim Broadbent, Hugh Grant and Joanna Lumley in a sketch for Comic Relief. None of them inhabited the part with a tenth of the conviction of David Banks.

But the most famous non-canonical Doctor of them all is surely Peter Cushing. After the success of the Daleks on TV, producer Milton Subotsky was responsible for bringing them to the big screen in the movies Dr Who and the Daleks and Daleks: Invasion Earth 2150 AD. The veteran Hammer Horror star Cushing was cast in the mistitled lead role. Now, these movies have their admirers – current producer Steven Moffat, for example – but as a child, I knew these films weren’t kosher and this guy playing ‘Doctor Who’ was emphatically not the Doctor; as well as having the wrong name, he was just too human. Besides, rather than laser beams, the movie Daleks fired lethal blasts of compressed air – in a big-budget film, for God’s sake! Like Milton Subotsky could give a shit.

* Quantity and names of actors who have played the Doctor correct at the time of going to press.