And just as a final follow-up, that mix had all leaked through into the crankcase by the following morning... (oh, and I cross-checked flywheel position against the 11HP engine, and no problems there, so not sheared key)
Sometime I'll pull the piston out and check the rings, but I'm not in too much of a hurry now - I dropped the 11HP engine into the mower at the weekend and then cut the grass, and it worked far better than the 10HP ever had and didn't go through any more gas doing it.
I might try and fix that 10HP one day though (assuming it just needs new rings rather than a re-bore) just to keep a spare around... or I might find a use for another engine around the place.
cheers
Jules