website which lists London bus routes just includes 400, as if there were no question: it runs from East Grinstead to Caterham-on-the-Hill and here it is; but it's not on the TfL lists or on the maps, not even under that arcane 'other bus routes' section. So we believe it is not 'one of ours'. The equally obsessive eplates website seems to suggest that it ran around Slough.
So we are just going to say that we shall not be travelling any 400, which is a shame, since you could call the number something of a milestone.
In fact, as we move through the 400s, we shall find that there are many non-existent routes, or unused numbers, whichever way you want to look at it. The Rules Committee, in an extraordinary meeting - well, exchange of emails, actually - has resolved that consecutive non-existent routes may be lumped together for blogging purposes, otherwise we might run out of things to say when we have not travelled.