Hmm. It's totally cooled down, and no apparent yoghurt in the oil (ie, emulsified water/oil) which would seem to say, no coolant. But as YC say, it shouldn't really be petrol- petrol
can blow past a broken ring but not in these volumes.
So... Having had a bit of time to think, the most obvious conclusion for that does seem to be that I've screwed up, and that maybe the bike was at more of an angle than I thought when i filled the oil, and just overtopped it. I
may have been having a bit of a flap when I posted this

I'm going to treat it as a loose clutch cover for now and see what happens.