Whew, that was a marathon and no mistake! (Well, plenty of mistakes on the way, at least at my end.)
Thanks to Paul for a masterwork, and to so many contributors here for the excellent help (Brendan and Chrise stood out among many) and the invaluable questions, which many times gave me pointers or at least assured me I was not struggling alone.
FOI 15a, slightly hesitantly; LOI 17a; lots of COD contenders but probably 8d just gets it.
I'm away so didn't see the paper this week, but much sympathy to those who tried to get this without the instruction, it was tough enough with all the help available! I missed the significance of A or I for much too long.
To add to the app discussion around @127, I think there are two flavours of all. My Guardian (news etc) app (both on Android & iPad) gives no indication of success or progress with the prize crosswords, although it allows checking or cheating on the other puzzles; I think the G puzzles app must give a tick but I haven't tried this one.
Thanks again to all, have a good week