Thanks for the advice, I will follow it!
Hopefully the isolating valves won't leak - I think they were replaced at least once since new. If they do they look easy enough to get at.
More of an issue is if the isolating valves let by - that is hard to spot seeing that there are pressure drop issues already!
My client / friend has reported that the pressure has remained the same for 2 days now, meaning that there is some intermittency to the pressure loss. She has been having baths etc as usual and using the heating during the day as normal.
I suppose there could be a pipework leak that is heat sensitive and therefore intermittent, but I suspect the boiler the more likely fault.
Thanks again,
Axel