i have the same problem. i have had baxi look at it and an independent engineer. apparently, it is most likely to be a dysfunctional expansion vessel and it is not unusual for this to need to be replaced after a few years. the parts are under £100-ish (I am told), but the problem is that the boiler needs to be taken off the wall. that is a good few hours work. Baxi were hopeless and i would recommend you avoid them - a great deal of money for terrible service. their solution was to slave a replacement vessel and stick it on the wall nearby. a cheaper solution if the boiler is out of sight. mine is in a bathroom, so i didn't accept their solution.
worth looking into anything else that might need replacing and getting the whole thing serviced (diaphagms etc) wilst it is down. will save you money in the long run. good luck!
sam100
March 2008