Britony Combi Se loss of pressure?

Hi, I have a Britony Combi SE boiler (an 80 I think) and I am losing pressure from the central heating side. I am fairly sure it is not a leak from the radiators or valves. I have noticed that when the hot water comes on, the pressure guage drops. This seems to me to indicate that somehow the central heating system is leaking into the hot water system. Does anyone out there have any ideas please?