I also have this problem. I belong to the British gas homecare plan and they do not seem to be able to fix it. I switch mine off at the mains for a few hours, then switch it back on. Sometimes this stops it heating up hot water when you don't want it to and sending to the radiators. It always reverts to the behavoiur after a while. Not much help I know... but saves your sanity for a little while if it works.
Milly molly
April 2008