OK very last attempt at helping you, do EXACTLY as I say, tomorrow, set the boiler pressure to 1.5 Bar, with upstairs and downstairs both open, turn the power OFF to the boiler dont bleed anything dont touch anything then go back and see what the pressure is a few hours after, has it moved ?
Hi, I'll get that done tomorrow but it's pretty much what I did today. Topped up the boiler to 1.5, turned off at switch and then went back to check the water in the measuring jug. The pressure was at 0. No radiators were bled, literally just turned the boiler off at the switch. It was round 90 minutes that I left it.