Thanks all, and good to be back - this is still the best rpg I've found for the android format, and I reckon I've played pretty much all of them.
I seem to have glitched it in that the area has respawned with the various monsters, including the named imp, and does so fully whenever I step out into the tower, though I'm not sure if the imp still has the crystal as I've yet to kill him again. I still have the option of the crystal in the snow area, so I'll be able to continue the main story without any problems. I believe what happened was I left the area at exactly the same time the monster died, and for some reason it didn't generate a corpse.