I have a Glow Worm 24CXI Combi that has been leaking water for the last 12 months or so. On Heating its fine, no leaks. The boiler looses no pressure at all. On hot water all is fine when you are running the hot tap, but once you turn the tap off water comes from the underneath of the boiler. As soon as you turn the main cold feed isolation valve on the cold feed off the leaking stops. This is how I have been running baths etc for the last 12 months. Someone has suggested it may be the PRV.