Ok. My guesses are: 1 you have a leak on the system (unlikely) 2 from the high pressure in operation that the expansion vessel and pressure relief valve need replacement. Put a sandwich bag over the prv discharge pipe outside. If it’s got water in it then bingo there’s the problem (v common) 3 the heat exchanger is pin holed and you are losing water down the condensate pipe which is why you never find it (rare as hens teeth) how confident are you in checking the expansion vessel your self?
You need to come north ,all sorts of biscuits here. I've had the southern rich tea malarkey that's why I moved north
Before I moved to Cheshire I used to think that Birmingham was far north I've since been corrected though more than once
