CH leak?

There will be a relief/safety valve on the system near to or inside the boiler. Tie a plastic bag loosely over the outlet to this and see if you catch any water. This may happen as the system warms up (water expands)

If so this is a pointer that your expansion vessel is knackered, ie the diaphragm inside has split and ther eis no longer space for expansion. You will then need a new expansion vessel.

Mine was similar, and turned out to be a weeping valve. Never became wet as such, as it got worse when hot and evaporated, so no puddles.

If it's not the valve, or somewhere in the boiler as per Harry's advice, you (just!) need to go around the pipework systematically.

I wouldn't be inclined to leave it too long. 1 bar is probably a good few litres of water. While it might be going somewhere quite benign, equally, it could be festering beneath a floor. And air in the system is not a good idea.

Can't comment on leak sealers, but in the absence of some decent anecdotes/evidence, I'd steer clear.

Thanks guys Thought more people might have experience re; leak sealers. Have taped a bag over (what I think) is the overflow. It was showing drips but the boiler had just fired so not sure. It was serviced about eight months ago so might have the Worcester guy back a bit early. Cheers Jb>

That's exactly when the drips would appear, as the system warms up. Looks like you expansion vessel is knackered. It may be inside the boiler and of a special shape to fit (hence expensive).

A standard (cheaper) one can be fitted outside the boiler.