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.