Anyway, yesterday The Husband got a phone call at 4.20 saying 'oh, we're coming round in a few minutes'. Which was helpful, as he was at work at the time. So he dispatched parents in haste (or possibly a Rover) and when we got home at 6 they said there'd been no sign. He finally appeared at 7pm. Fitted a new circuit board, promised it was fixed, and went.
He came back. At 9pm.
He left again at 10.30pm, confessing self stumped as to cause, declaring various bits in there don't appear to do anything at all, and that it appears to be missing a vital bit common to most other boilers, only there doesn't seem to be a place for it (it's not especially old or anything). So he's going to speak to the other engineers on Monday, at which point the whole sorry saga will presumably start all over again. *headdesk*