if it doesn't do the same in central heating mode, I'd say the DHW Thermister. have a look in the manual to local it. If its in a wet pocket you'll need to turn off the main cold water feed to the house, open all the hot taps to drain the water, and be ready with a tup to catch a little water that will come out of the pocket. if its in a dry pocket, your langhing, a smurf could change it. Just MAKE sure the electric is off before you put your hands in the boiler. REMEMBER....ELEC OFF/FUSE OUT. EVERY TIME.
LTGF, August 2008