how to fix potterton performa 24?

boiler continues to burn,bypassing all instruction to do otherwise.ive tried resetting but again the boiler just keeps fireing up and burning which in turn starts the central heating.the system will not disengage.the only way to stop this is by switching off the unit completely.please advise.also is there any way to isolate the central heating from calling allowing the the hot water still to operate.