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Well, he came, he fixed it, he went.
'Something round the back to do with pressure regulation' was as far as he'd divulge his secrets, but he stripped it down, did what he had to do, and it now works fine, so all well.
One thing we didn't know (oddly) was that we're covered not only for the boiler and its bits, but also for radiators. They seemed to have added it to the premium policies over the years without telling the customers about it. He looked at his screen, said 'you're covered for radiators', we double-checked, and damn me we are. Not underfloor pipework, of course, but nice to know about the rads.
Anyway, that came up because we had a leaking radiator, which he's now ordered a replacement for.
BB