I'll have to wait a while until I can borrow a compresion tester.
In any case, I took off the carb today and checked it over, cleaned it through. It all looked fine, but I will test it at some point.
It's running fine at low revs, but once you get to about 20% throttle it sounds like either fuel starvation or running too rich (fuel-air, not fuel-oil). It's odd, because it was working fine and I haven't changed any carb settings.
I've checked the fuel pump and line, it's all getting through to the carb, I've checked the carb and it's all getting through there, it's new fuel and oil.
I'm just at a loss as to what else it could be!