Thanks Bizzard,
I went back to the engine a few days ago and found that it has a compression problem on the cylinder closest to the output shaft. When I started it again, the cylinder wasn't getting hot at all, I can also turn the engine over with the 'good' cylinder decompression on, though not easily.
It has aluminium push rods, and I measured the clearances at about 0.35mm cold. Bopping the valves didn't make any difference.
With the fuel pump window off (which had sellotape on a homemade cork gasket!?), it sounded like the compression was escaping through the crankcase, I've taken off the heads and found a right mess in the cooling tins, so no wonder it was overheating.
I can't see any scoring in the cylinder bore, I'll see if the valves leak fluid tonight, but it's looking like I might have to take the piston out.
I did discover that the cap that screws onto the top of the injector was loose on the affected cylinder, and there was fuel between the cap and the large slotted top.
It then started raining, and since I'm doing this in the middle of some woods, I left it at that for now.
Serial number for those playing along, is 4610SR2Z17, 14.5HP, 2200RPM.
The puddle of fuel in the good cylinder is I think from where I was turning it over a bit.