Thank you for that. Having done some tests looking at the spindle with the actuator temporarily removed and comparing with your notes I now think the valve is working correctly. This is confirmed by feeling the outlet pipes from the valve getting hot with the boiler and pump running and with different demands set. I now suspect the programmer / timer (Danfoss FP715) which seems to turn itself off with a blank screen from time to time and when it comes on again all the times need resetting. I am still not sure how this would make the hot water so hot. The other suspect could be the water thermostat on the cylinder. Any views on this?