Apparently, at the end of St Elsewhere
Here is a link to one of those internet sites:
Also, on the key, it says HHGTTG is part of it. My memory is fuzzy, but I think the Doctor only mentioned him in passing, because he had just saved the world in his dressing gown. So we can scratch one off.
I apologise if any threads have been made on this. I looked it up and couldn't find any, but I may just be incompetent.