I've replaced the control unit and the problem persists.
Hot water responds to timer etc but central heating stays on constantly and has to be controlled by the thermostat or isolated altogether at the mains.
The PCB is less than 2 years old ...Has this gone again?

Ray King, December 2008

Boiler is operating as it should,you have a system fault
Check programmer room stat and diverter valve
Check if you have a frost stat if calling it will override system control

a, December 2008