is it repairable?

I have a glow worm swiftflow combi boiler and there is a constant drip coming from the over flow outside.
The heating still works, all radiators are fully hot, the hot water works at a decent pressure.
The pressure on the guage will slowly drop until we refill it. I think it is possible the pressure release valve that is stuck open slightly. If this is the fault are parts available for this boiler. I think it is the 100e model, but not sure, it is 12 yrs old. thanks for your help