Problem with Potterton Promax HE24 Boiler?

The hot water system works ok. I have fitted a thermometer to the outlet from the boiler and this goes up to over 70 until the cylinder thermostat cuts in. If I run water then it cuts in as expected.

The problem is with the central heating.This cuts out before the room thermostat setting is reached. The thermometer reads just over 60 when it cuts out. After about 20 minutes the temp drops to 40 and the heating will come back on with the re-set button on the boiler.
I have fitted a new room thermostat, a new boiler safety thermomostat and a new thermistor. No change. I now wonder if it is one of the controls boards, but as these are rather expensive I am reluctant to do so without proof they are the problem.
Anybody with any ideas.