No the 3/4 brass nut that connects onto the vessel, this will allow the vessel to be lifted out for inspection and cleaning , not a big job to do, you will be surprised bow cruded up it will be in there, and as already said have a poke upwards to make sure that is clear too
Hi Ian,
An update... I removed the vessel yesterday, which was a very simple and straightforward task. The weight of it, full of water, was quite a shock! I'm amazed the wall bracket hadn't failed. Anyway there was actually no blockage the water tipped out unimpeded and the adjoining pipework was also completely clear. When empty, however, I still couldn't get any air into it even with the Schrader valve removed. I pushed an Allen key through the valve hole and could feel the rubber of the diaphragm, it was stuck hard to the vessel on the inside. I think the vessel must have failed a very long time back due to lack of maintenance and I hadn't noticed.
Anyway I duly fitted a new one and everything is fine. In fact it's better than fine, it's like a new system. The pressure, hot and cold, is spot on and the system is running more quietly and more efficiently than I ever remember. We live in a 150 year old house and I'd always regarded the knocks and gurgles in the radiators as one of those things you get in old houses, but they're gone now. It's runs quietly and the house is warmer too. Brilliant!
A big thank you Ian for giving me the confidence to tackle this job, I really am delighted with how it's turned out.