potterton suprima cutting out?

Sounds like either-
1) Lack of water in system, - check header tank ball valve, or if pressurised fill to 1-00 bar
2) Air in system , with heating & hot water switched off, thoroughly vent the system
3) Faulty Thermistor causing overheating

If you have the new style board ands wiring loom it is unlikely that the board is faulty

Boiler Repairman - Ipswich
April 2009