Update:

Fitted a new ISCV, but no change. Worth mentioning that the fault is becoming more regular - it seems to play up after around twenty minutes, so it's obviously heat related.

It has settled somewhat - it will idle at a lumpy 500RPM alot of the time. I do think I have an air leak though, but I have checked with a can of WD40 etc, since I can't set the base idle.