Quote to diagnose combi boiler depressurising

I have a combi boiler which is 7 years old and for the past few weeks has been losing pressure over the course of 2 days or so. It only seems to lose pressure when its being used. No leaks from any pipes or radiators. I have bled all the radiators and topped the system back up - same issue.

Our usual plumber came round and inspected the boiler. He then quoted me £700 for half a day's work to diagnose and potentially repair the issue. He said he would need to isolate the boiler and test and keep popping back during the day to check it. He then went on to say it might be better to just buy a new boiler, that he would install it etc. He's been very trustworthy for the last few years that I've used him, but this felt really off.

I have done some research and a lot of people seem to suggest the expansion vessel or pressure release valve might be at fault. I asked the plumber if he thought these were worth replacing and he said he couldn't be sure without the half day's diagnosis.

Any advice?