Coldest night of the year, maximum butt factor, but how did it know when to fail and cause the biggest problem?
Anyway, got onto t'internet, found a schematic, stripped controller board and replaced three capacitors; had to go to two different Maplins (one twice, 'cause they gave me the wrong bloody one)

Back to working now, though.
Saved meself a huge callout and bill for new parts, anyway, so all good in the end.