I think you might have a problem with the central heating flow sensor, it tends to cause problems with this boiler. it is magnetic, and guess what, so is black deposit in central heating water. You need to isolate water valves to central heating, depressurise boiler, remove and clean sensor being careful not to lose small spring.clean slider and shaft. be aware there are two flow sensors, one on left is the problem I think. cleaning might not work, so a new one is next!
best of luck.
Ian Cockburn, December 2008