Aha! I think you're right. Just thinking about it now, I was running the car quite hard with acceleration and at high speeds and then pulled over for 5 mins and just let the car idle while I was talking to a mate. At this point everything was fine, then jump in car, accelerate hard and find poor power and so on.
So probably after giving the car a hard run, then pulling over and without airflow through the engine bay it was overheating while I left it idleing. Does this sound like it could be the case?
And further again about a head gasket, is it a difficult job to fix/replace and would it take long for a mechanic to do?
Thanks.