I didn't write it down at the time, but I'm pretty sure that this is the error the boiler gave last time it died in November 2013, when the gas engineer fixed it by drying the whole thing out with a hair dryer. It means the primary heat exchanger has died (major part), or possibly, that a connection to the heat exchanger is playing up. Fingers crossed. I may try the hair dryer again. But can we possibly need *another* boiler? Surely it is unusual that our boilers fail so regularly.
If we have to replace it, maybe we will try getting one from Calor this time. Calor sell us our LPG, so they really should know which boilers work best on LPG. It's not *that* unusual a fuel, but you really pay through the nose for not being on mains gas - not just for the gas deliveries but fires and boilers etc seem to be more expensive and harder to come by.
At least our recent negotiations to get some solar panels installed have turned up the fact that the hot water tank does have an electric immersion heater which I'd previously not noticed. So with a bit of luck we will at least have hot water, even if we can't rely on the sun to provide the power for it yet.
ETA: the boiler is fixed! Tried an exciting new plumber, a Cornish one rather than a Devon one, name of Artifice Plumbing. He came within 2 hours, twiddled the pump to release an air bubble, and it is working again. Hurray! this weekend's task: bleed all the radiators... :-(