Well, managed to coax the boiler back into life, maybe we'll get one more winter out of it yet....
1st problem was the pump, which had failed short-circuit. Changed the pump head for a "new" one (actually, one pulled from another boiler), after some swearing & cursing brought on by a stripped thread, the silicone glue was administered to seal the bugger up.
2nd & 3rd problems: The pressure relief valve (PRV) was widdling water all over the garage, and when the boiler lit up the pressure in the system simply rose until the safety system cut it off. Turned out it was suffering from the automotive equivalent of a flat tyre. Cure is the same: Pump the "tyre" back up - in this case, an air bladder inside an expansion tank. Add a few PSI, drain off the now excess water, add a bit more air, drain a bit more water, and presto, the water pressure no longer skyrockets as soon as the boiler kicks in. Result! Brucie bonus, the PRV valve stopped dripping too.
4th problem, the gas valve micro-switch seized again (3rd time this year, I should probably use some proper oil or grease rather than WD40

), resulting in the boiler thinking there is no gas pressure; cue a safety shutdown. Disassembled, WD40'd, reassemble, and finally all is well again!
In fairness, I'm glad the boiler shuts down, rather than exploding, bursting the CH system, or bursting into flames; but it's still a PITA when it happens.