The pressure vessel will have lost its charge. (They always do) with no pocket of air in the system, it'll only take a tiny amount of water to leak to drop .5bar.
The vessel will need recharging to .75 bar. (While the boiler is empty & drain offs open) the pressure sensor will need cleaning & you're also well overdue for the burner seal to be changed. Oh, & the pressure relief valve will need cleaning.
Hope this helps. It'll take an engineer about an hour all in all.
Thanks. I like that diagnosis. So paying £300+ sounds like it would be more than I need to pay.
How much should I be paying to have this done and can you recommend a good not too expensive gas safe engineer for me? I'm in London too, north London.