Ravensheat RSF 100ET - No Hot Water?

I have a Ravensheat RSF 100ET Combi-Bolier. The combi-boiler manages to heat up the radiators fine, but recently the hot water is hit and miss. It seems to heat up the water when the boiler hasn't been on recently, but if the radiators are getting heated up there's no joy and no hot water at all. It sounds as though the boiler isn't firing up the gas to heat the water, but I won't pretend to have any in depth knowledge on these things. The pressure reading is round about 1 bar and I'm sure it used to be higher. Can anyone give me any pointers on what might be wrong - or should I just get someone out to service it? Thanks.