Hi everyone, well I reset the clearances and it seems to be running a bit more smoothly now, but the idle isn't much lower (I haven't tached it yet tbh, British weather is preventing me doing anything outside!). I'm going to buy a 12x7 prop today and throw that on it and see what it gives. The valves weren't out by much, but I reset the clearances anyway.
It will idle smoothly at the current setting for about 30 seconds then slowly slow down, eventually seeming "misfiring" a few times then eventually stop.