A few days later I picked the film up from Boots to find it was completely blank! This puzzled both me and the lady at Boots (who was kind enough to not charge for processing) - the film was not obviously defective as the leader was exposed, and being a SLR it would have been rather obvious if I'd left the lens cap on. However I've come up with a theory: the Nikon doesn't have a slot you have to thread the film through, but rather you pull the film out far enough to reach the takeup spool and it grabs the sprocket holes and winds it on automatically. Except if I haven't quite got things lined up then it won't do so - and the symptoms of "film pulled all the way out of the canister" and "film not advancing at all" are identical.
Lesson learnt: pay attention to the telltale spinny thing when loading a new roll of film, and check that it is actually advancing properly!
