The boiler serviced, we learnt that the anti-corrosion gadget on the pressurised water cylinder, had itself corroded.
Two weeks later, the small ceramic spigot - mounting for a titanium rod - and costing £87 ... arrived with the plumber. He shared our lunch since he was delayed and we welcome him here! ...and then set about removing the original anti-corrosion gadget. Which had welded itself to the cylinder. An anxious Martin appeared, headlight in place, to alert us to the problem, he was having to drill it out - bit by bit.
Having a remarkable husband, who has almost everything anyone could ever need - from books on early boat-building to tiny tools suitable for unscrewing broken ceramic gadgets - by some miracle he found his tools straight away, tucked under the flap of one of the mountain of cardboard boxes in the garage. The gadget was unscrewed in due course, and the new one placed - water on, problem solved.
Martin's fear had been that the small ceramic remains would not come out, and then a new cylinder was the only option: not a welcome thought - thank goodness for the tiny tools!
Why do we need such a gadget? well, pressurised cylinders cannot be left to corrode - reaction between different metals - and risk an explosion. No-one asks why, in this case, there should need to be different metals, if they are known to react. There is no answer, probably.