Great Guys thanks, First the thermostat was replaced to fix the problem so its highly unlikely that that is the problem. Yes the cap is suspect and will be replaced next, however the intermittant thing just dosen't seem so simple (I hope so and will do it). Next I assume the consensus is that the heater valve is ruled out as a culprit so Finally what and where is the monovalve and can it cause these sorts of symptoms??
Thanks again in advance
Alex