Well done the local fire-station, who got involved very quickly - they're just up the road - via Brother-In-Law Fireman Rob, mostly as a result of us drawing a blank on all the sparks we know. Yah Boo Sucks to Southern Electric, who came out and pronounced it 'Not our problem' - but then pulled the main fuse 'For your own safety', 'Can you recommend an electrician?' 'Sorry, not allowed, totally against the rules Guv'.
And thank-you Rob, my sister's neighbour who came to have a look. And thank you LOADS to his boss, Jason, who turned up this morning, drove to Cheam for an RCD, came back, fitted it very quickly, dead-tested the whole house, all at very little notice. Thank-you even to the second Southern Electric chap, who smiled just a little when he came to put the fuse in and manage the 'Let there be light' moment and wasn't a complete t***er like the first one was.
Much kudos to Jastone Electrical and the gents from Wallington Fire-Station. And remember folks: Old houses have old wiring and old wiring can misbehave. I'm lucky my house is still here to be sat in.