The boiler is fixed! Hot water!
Celebrations aside, minor domestic situation in my house as the engineer made no secret of the fact that we’d caused the problem ourselves and she shouldn’t have been called out. Wor Lass took exception to her tone and had to be escorted to the living room while the engineer drained the boiler and fitted a new pressure valve. She was a bit abrupt but I had a feeling that might just be personality rather than anything personal.
I’m now trying to defuse Wor Lass, who has had a hard three weeks, but don’t really want to get drawn into the fact that the reason the heating broke was that she had been mucking about with the pressure taps beforehand.