Sorry folks, have to make a brief return from exile here:
tilbee, just before my time as a serious solver - though I remembered seeing this one and marvelling at how on earth people knew this stuff at the time (I don't think I cracked more than a handful of clues).
Check out Listener 3560 ,Trainspotter by Dimitry (= ex editor John Grimshaw) from the year 2000 which clearly predates your comment by 11 years ... solution summary reads
"Subsidiary indications in clues involve extra letters, spelling GRYF-POTTER-FINDOR, SLYTHERIN, RAVENCLAW, HUFFLEPUFF; unclued lights down are all trains, including HOGWARTS EXPRESS from platform 9¾ (which grid number is therefore to be entered)".
Incidentally, one of the prizewinners seems to have been a certain Derek Harrison, master of the Crossword Centre site.
Good ol' northern Grammar School
ciao