So, a quick look around, no suspicious characters.
However, a little while later, meowing roused me again. I checked downstairs, couldn't see anything in any rooms with open doors.
Then, I checked the downstairs cloakroom.
Kosh (our Silver Abby tom) was in there. He'd pushed the door open, then closed it behind him to play with his prey in peace. Sadly for him, although he seems to have mastered the art of getting in, he can't actually pull the door open again, and had trapped himself. So I let him out, retrieve the corpse, and dispose of it, before wandering out for a quick smoke beside the pond.
A minute or two later, out he wanders, and starts to lap at the water. He does this for quite a while, before pausing, with a somewhat startled look on his face.
What he hadn't noticed was that one of the fish, a fair sized one at least six inches in length, had been attracted by the splashing on the surface and had risen like a trout to a fly. Which was more surprised - the cat to have his tongue grabbed suddenly, or the fish to discover that it had tried to catch something far larger than it, I can't begin to guess.