Ok, so at least I know what the problem is. We have a leak. The leak was not showing, no damp anywhere?
Some numpty (presumably the original plumber when the house was built) has put a push fit elbow on one of the CH pipes (10mm microbore poly) inside a stud wall, between the GF wc and the utility room. The elbow is leaking inside the stud wall. Hence the boiler pressure drop.
We called in a boiler engineer and as soon as he topped up the system we both could her loud running water p*ssing out, we immediately went to where the sound was coming from, eg: wall in utility. I think I did not hear water before this as it was a small leak, dripping maybe, but when the boiler guy came, I think he topped the pressure right up, eg; 2 or 3 bar (whereas I only top it up to just under 1 bar) and this extra pressure made the leak worse, so we could hear the water. He could not do anything apart from drain the system and turn off boiler to stop the leak, (eg: heating guy, so did not want to start pulling out utility sink, worktop, make holes in wall, etc, etc) and advised to use insurance.
So we lodge a claim with insurers (on Monday). Its now Friday, had no HW since last Friday, and its still not fixed. [Sigh!]
1st guy came on Tues, emergency plumber, despite me telling the insurers the water was off an no current leak. He came, looked, said sorry mate can't do anything else you need the trace and access people to find the leak.
2nd guy came on Thursday, trace and access guy, he cut hole in the wall, put his camera down, connected air line, found the leak, but said he only finds the leak, he does not fix it, thats another company.
3rd guy is due today, a heating engineer apparently as they can't use a normal plumber cos its connected to the boiler. He's due anytime up to 5pm today?
So, so far its been a week without hot water and the leak has been found, but its still not fixed.......[sigh].