Thanks Demeter. This was one of those Homer Simpson moments when you realise your own stupidity! Haven't used a 25A since I was a child. Which leaves only 1A and the 15-letter word. Only...!
Thanks Carpox! I vaguely remember a previous use of the word in that sense in a Listener, but I would never have remembered it without your hint. Still the 15-letter word...!
I can see a method, I have a 15-letter word that meets these criteria, it matches the title - but it is not in Chambers (that I can see). Do I need to keep looking?