He also said that sometimes the ‘overfill’ trips so I’d need to bail water out of the brass expansion chamber in the top.
I find that the overfill switch is tripped during the night and I have to bail the water out of the expansion chamber before I can take a shower. Even then the same problem applies.
I haven't seen that cylinder before, interesting.
There must be a reason why the overfill switch is tripped and you have to bail it out of expansion tank. The overfill switch, when tripped, also closed off the cold supply to coil for hot water. I reckon you have a pinhole leak thro coil so cold supply leak into heat store and raising water level up to where it's tripped the switch which stopped to cold water coming in by that shut off valve.
There a way if it is a pinhole, remove the flex pipe attached to valve with blue knob and hang it up and tie it. If the overfill tripped again, you got a pinhole leak.
Daniel.