I had the same expansion vessel issue on my combi
Tap it with a spanner it should sound hollow if not the bladder could be fugged and the vessel is full of water
Alas it was a drain down job to replace took me about 1 hr including refilling the system in a 4 bed house
Got my vessel from here http://www.heating-parts.co.uk/
cheapest I could find
DO NOT CALL IN BRITISH GAS THEY WILL ONLY TRY TO SELL YOU SOMETHING ELSE OR TELL YOU YOU NEED A NEW BOILER TO BOOST THERE “QUDOS” POINTS