Hi Kim... Andy Grangers old one?!!
Dont suppose you have had the rocker cover off have you? The slightest kink in the gasket on refitting with make the car run like a dog! Also, check the breather pipe going from the right hand side of the rocker, down to the block (crank case breather).
In fact, take it off and blow it through as these are common to block with ....crap basically!
As for the heating problem, I think Andy did a headgasket before on it? I think I got that from the RSBB. Anyway, obveously no sign of crud in the oil cap or on the header tank?
Huw