From the outside, Doctor Who’s time-travelling machine The Tardis appears to be a small and rather battered 1950s police box.

Closer inspection reveals it to be vastly bigger than imagined and bursting with possibilities, a neat metaphor for the phenomenon that is the Doctor Who franchise.

It is almost five decades since William Hartnell appeared on TV screens as the Time Lord and since then, the format has garnered millions of fans via TV, film, books, magazines, audio tapes and DVDs.

Matt Smith, current torch-bearer, is the 11th actor to play the role and the latest in a long line. Thanks to the Doctor’s ability to regenerate, he has morphed his way through Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, Christopher Eccleston and David Tennant.

Science-fiction and fantasy author Juliet E McKenna, a life-long Who fan, is looking forward to exploring the reasons behind the Doctor’s enduring popularity when she chairs a panel at the Oxford Literary Festival next month.

The session, ‘Doctor Who: It’s Bigger On the Inside’, also sees the launch of four Doctor Who children’s books — Death Riders, Heart of Stone, The Good, the Bad and the Alien and System Wipe.

Witney-based Ms McKenna will quiz Who authors Justin Richards, Oli Smith, Colin Brake and Trevor Baxendale on plots, monsters, companions and of course, the enigma that is the Doctor.

She said: “I shall ask each of the writers who was their first Doctor, because I think that defines your relationship with Doctor Who. They have written four very different stories, so I want to know what sparked each idea? How did they find working within the guidelines?”

She is referring to the fact that anything written for tie-in fiction, such as Doctor Who books, becomes part of what is known to fans as the ‘Whoniverse’.

“I will also be asking them if there is an iconic monster they would really like to write about.”

For her it would be the Sea Devils, scourge of her childhood. “Those monsters made a real impression on me when I was young. The idea of something unseen lurking beneath the water was terrifying but fascinating.”

She was also delighted to see the Autons, life-size androids with an unfortunate habit of killing humans, making a comeback in the relaunched TV series six years ago.

“When I was a child, my mother used to have to drag me past Littlewoods’ shop window, I was so frightened of the shop dummies,” she admitted. Alongside 12 original fantasy novels, she has written short stories for Doctor Who — one set in the Bodleian Library and another centred around Who spin-off Torchwood.

“One of the guidelines given when writing for Doctor Who is ‘he is never cowardly and he is never cruel’, which is the essence of a hero, if you think about it,” she pointed out.

“He is an individual who has always stood up against unthinking authority. He never needs to apologise for who he is or what he does.

“I suppose the main message is that you do not need to accept what you are told without thinking it through. You do not have to follow the herd.

“The Doctor is a great problem solver. He uses lateral thinking and I think that appeals to young people,” she added.

Her first Doctor Who memories, although vague, are of Patrick Troughton but it was the third Doctor, Jon Pertwee, she bonded with.

“The thing I took from Doctor Who early on was the infinite possibilities for storytelling. You can set your story in any period in history, anywhere in the universe. There are no limits.

“It can be the huge, big-picture stuff of asteroids exploding or intensely personal, specific stories and that’s the strength of the format.”

She believes that children’s relationship with story is completely different today from when Doctor Who was first broadcast.

“Kids have a more sophisticated understanding of story and character than we did.

“The standard of the TV series is now very high, so the bar for tie-in fiction such as Doctor Who books has to be as high, if not higher.”

* Death Riders, Heart of Stone, The Good, the Bad and the Alien and System Wipe are double-ended books published by BBC Children’s Books at £6.99 each.

Doctor Who: It’s Bigger On the Inside, a family event suitable for ages seven-plus, is at the Oxford Literary Festival on Sunday April 3 (see www.oxfordliteraryfestival.com, box office 0870 343 1001).