I decided to re-fit the heater cowling. New thermostat was purchased but to my horror it is not opening!
This is the second one I've had and loosing the will with them thus I have a plan.
I was thinking of a cable operated flap.
I'll simply keep it open in summer but could aid the heater [-/
The only thing I can see for this is to get a fairly accurate temperature gauge working then regulate it on that, surely this has to be more effective!