this is probably because the boiler thinks there is constant hot water demand because the domestic hot water microswitch is broken.
Try turning down the hot water temperature control, does it modulate the central heating too now?, also switch to summer only mode -the CH should be inoperative, but will stay on with this fault.
Cheap and easy to fix - get a new microswitch and wire it in. Replace the domestic hot water diaphragm while you are at it. Disconnect the mains first as the switch is live.
c
February 2010