Christian McArdle wrote:

Sounds more like the expansion vessel has lost it's air volume.

It could be that the diaphragm has burst, and the thing is full of water,
or it may just need re-charging via the schrader valve.

Yes, I didn't read the OP's post thoroughly enough. I would agree that this
is a more likely scenario.

Christian.

Thanks for the advice, I'll look at that. I've noticed another
problem, which may or may not be related, which is that the air-vent
cap, which should be left loose, leaks water when left in this
position. On the matter of the expansion vessel, how much would it
cost to replace? It's a Potterton boiler and the manual advises that
the replacement should be external to the system. What do you think?
Again, your help and advice is greatly appreciated.