10 things you didn't know about Doctor Who

1: Education, education, education

Doctor Who was originally meant to be an educational programme for children, rather than a complex sci-fi drama series. Indeed, the co-creator Sydney Newman expressly stated that there were to be no 'bug-eyed monsters'.

The stories set in the past were to teach kids about history, while the space-bound tales were to provide bite-sized facts about science. This is why the Doctor’s first two companions were a history teacher and science teacher.

All of this changed with the second story, which featured the Daleks and made Doctor Who a sensation. After that, it was bug-eyed monsters all the way!

2: The ultimate get-out clause

The concept of regeneration, now such an integral part of the Doctor Who universe, was only invented because William Hartnell was too old to carry on playing the role.

In fact, Hartnell was almost written out much earlier. The production team briefly considered replacing him with a younger actor in the story The Celestial Toymaker, in which the villain renders the Doctor invisible for a time. The original idea was that when the Doctor re-appeared, he’d have a new body.

If they’d done this they might very well never have come up with the idea of regeneration, and the series may have ended with that younger Hartnell replacement. As it happened, Hartnell had a bout of ill health in a later story, forcing the BBC to quickly replace him – and invent the idea of regeneration to explain it.

3: When tragedy struck

Tragedy befell the series during the Jon Pertwee era when Roger Delgado, who played the Master, died in a car crash in 1973.

The character of the Master had actually been scheduled to die in a story called The Final Game (which would also have been Pertwee’s final appearance). Just before filming began, Delgado went to Turkey to shoot a comedy film; while there, he and two film technicians died when their car veered off a road and fell into a ravine.

This meant that the Master simply vanished rather than died, and the role was later taken over by the actor Anthony Ainley.

4: Do Time Lords Fall in Love?

There was much speculation at the time about whether the Fourth Doctor (Tom Baker) was romantically involved with his companion Romana II (Lalla Ward). In actual fact the actors had a whirlwind romance while filming the series, and ended up marrying in 1981.

Yet it wasn’t to last, and they divorced just 16 months later. Lalla explained that they "just irritated each other", although they remain close to this day. (Lalla, by the way, has since married the biologist and 'pope of atheism' Richard Dawkins.)

5: Tom’s troubles

He may have played the Doctor the longest, but Tom Baker had a pretty rough time of it during his tenure. Just before shooting one of the seasons, he was mauled by a dog and his lip was temporarily disfigured – meaning they had to work the wound into the show by having him bang his face on the TARDIS console in The Pirate Planet.

Later on, Tom suffered a mystery illness that left him skinny and lethargic – and even led to his famously curly hair going straight. As his unruly locks were part of the Fourth Doctor’s image, he had to have his hair permed for the show each day!

6: The hitchhikers guide to the Tardis?

The late, great Douglas Adams was a Doctor Who writer for a time, and in fact penned City of Death, widely regarded as the greatest Who story of all.

Featuring Tom Baker at his most Tom Bakerish ("I say, what a wonderful butler, he’s so violent!"), the story is sublimely funny – which is a surprise as the author of Hitchhiker’s Guide to the Galaxy had serious trouble with it.

Famously prone to writer’s block, Douglas Adams had to be locked into his flat by script editor Graham Williams, who plied Adams with black coffee until the script was sweatily rattled off in a mere weekend. And what a script it was.

7: Doctor versus Doctor

Colin Baker, who played the smug Sixth Doctor, actually made his first appearance in the show as a baddie during the Fifth Doctor’s era.

The story in question was Arc of Infinity, in which Colin Baker played Commander Maxil, a trigger-happy security chief on Gallifrey. He even got to shoot the Doctor! Indeed, it was his aggressive performance that made producers think he’d be an excellent Sixth Doctor.

8: Sylvester steps in

A dispute between Colin Baker and the BBC meant that Seventh Doctor Sylvester McCoy actually had to pretend to be Colin for the Sixth Doctor’s final scene.

Colin Baker’s tenure had come to a sticky end when the show’s collapsing ratings led to it being put on hold. Believing that he’d been badly treated by the Beeb, Colin refused to film his regeneration scene when the programme returned.

The solution? Put Sylvester in the Sixth Doctor’s clothes, plonk a big blonde wig on him, and use 'special effects' (some blurry colourful splodges) to conceal his face as he regenerates. This makes Sylvester McCoy the only actor to play two different Doctors on screen.

9: Lots of planets have a North

It was a big coup for BBC Wales to get a major star like Christopher Eccleston as the Ninth Doctor – particularly as he’d very firmly passed on the chance to play the Eighth (who was eventually portrayed by Paul McGann).

"I was asked to audition for the Doctor Who TV film six years ago," Eccleston revealed. "It was a definite no from me as Doctor Who hadn’t resonated with me for a long time."

So why did he want to play the Ninth? He just wanted to work again with Russell T Davies (chief writer of the new Doctor Who series), with whom he’d previously worked on the drama The Second Coming. And so the Northern Doctor was born.

10: Accidental inspiration

Torchwood, the new Doctor Who spin-off, got its name thanks to TV pirates - the real kind.

When the Eccleston Doctor Who was being filmed, the producers were so desperate to prevent preview tapes being stolen by pirates that they took the security measure of labelling the tapes 'Torchwood' – an anagram of Doctor Who.

So when Russell T Davies needed a name for an alien-fighting organisation in the series, the answer was obvious: the Torchwood Institute.