GN: The most local repair person called me back, can come tomorrow.
Not sure how it'll turn out yet news: I've got to go and get the PCB that I've diagnosed is the fault today. I'm reasonably confident that "PCB burnt in the corner" will be at least part of what is wrong. If there's anything else wrong it probably won't get fixed tomorrow.
GN: Sun's out and the house is warming up!
BN: As well as the PCB one of the connectors had melted enough to be declared unfit.
More BN: Repair man didn't have one with him. (In all my dealings with various boiler install / servicing / repair types the electrical side has never been their forte. But at least he didn't cut the wires to the thermostat or tell me that the extra resistance of thicker cables would be a problem.)
GN: He's coming back Monday, hopes he has one in his store.
Hmm news: If he doesn't he started talking about new wiring looms, which strikes me as overkill. I've just found the bit on Farnell, its an 8 way * 1 row 0.2" molex.
BN: Maplin don't seem to stock them otherwise I could get one tomorrow.