OK, after A LOT of faffing around trying different things, I finally managed to find an old version of the bytecoin wallet, import my .keys file then export a .wallet file which the newest wallet finds to be compatible.
Hate it when devs do this. I don't want to spend hours fixing things just because I left the scene for a while. Why was my old file totally incompatible? It shouldn't have been.
Anyway, seems to be fixed now.