Having stripped it, and looked closely, I think what happened is it has lost coolant, the MC18 head water outlet is smaller than the MC16, and I think it has got loose and lost the coolant - DOH!
Although the temp guage didn't go up much, it was about 1/3 up the guage. But the exhaust side of the piston was melted away.
It's got standard airbox and jets, so I don't think it was too lean.... I'll check though....
Cheers, Mat.