I've had this same ongoing problem with my boiler since June, Engineers have come out and changed a number of parts only for other problems to arise. The engineers I called had no clue as to why it would come on by itself, a part which they said MAY cause this is the diverter valve. I've had this part changed but other problems have arised so we are not sure if it worked.
At the moment my boiler isn't working at all and Im waitng for an ARISTON engineer to come and have a look at it but my regular engineer is looking to get a new boile as almost every part has been renewed.
Mel
November 2006