gloworm boiler

I can't see any leaks.

Looks like a call for the man.

Expansion vessel fault....possibly

Expansion vessel isn't working. It should be half full of air, separated from the water by a diaphram. On the air side, there's a tyre valve which is used to pump it up. Take the cap off and try releasing a little air. If water comes out, the diaphram has split and you need a new expansion vessel. If air or nothing comes out, then you can try pumping it up with a bicycle pump. With the pressure released in the heating system, pump it up to about 0.5 bar (7lb/sq inch). If it leaks out again, it might need a new tyre valve, if it's replacable.

Thanks. I've checked for leaks elsewhere and found none so what you say is probably the fault. Still have to get a man in. My eyesight and health isn't good enough anymore to risk messing with boilers in situ.