Synopsis: Sean is writing a Doctor Who novel for Virgin's Missing Adventures range of books. The only problem is, he can't come up with a decent ending. The Fourth Doctor's TARDIS materializes, and out he steps, all teeth and curls. He accuses Sean of having avoided him for so long that of course he's having difficulties writing a Doctor Who novel. Sean at first denies the accusation, but when he finally admits to letting his relationship with the Doctor pass into something like a childhood memory, the Doctor offers him advice on how to finish up.
Notes: Though this clearly isn't a parody, it does present canon challenges.
If you want to put this into your own personal canon, the most straightforward conclusion would be that the Doctor is a "real" person in our universe, who happens to have inspired a great deal of fiction. This is an easy, understandable solution which has been used to explain away the two Dalek movies of the 1960s.
Alternately, you could adopt a more complicated view, whereby Sean Longcroft is the "Gardner Fox" of the Doctor Who universe. It's possible, I suppose, to believe that Longcroft lives in "our" universe, but writes fiction about the universe in which the Doctor lives. This would then likely mean that the TARDIS has an ability to cross into parallel universes with some ease.
The problem with this viewpoint is that the 2006 televised season has revealed that "parallel universe hopping" is not something the Doctor can do at will.
For your sanity, it's probably best not to try to find a place for this in your personal canon.