Combi 80, CH losing pressure quickly?

Hi, just found this website and it seems there are a lot of knowledgeable people in cyberspace.

My question...

I have a Potterton Combi 80. The pressure drops from 3 to below 1 bar within 24 hrs under normal winter use, ie CH on for approx 8 hrs in 24, DHW for bath, shower, sink etc. I top up the CH system nearly every day at the moment, as the pressure drops to below 1 bar. The pressure relief valve seems ok, I've put a bucket outside and it doesn't seem to be collecting alot of water. I guess I must be losing about a radiators worth of water a day, as there is only one rad that doesn't get hot when the CH is on.

I can't see any pipe leaks, could it be the diverter valve or expansion vessel?

Many thanks