Found the problem - self-inflicted as so often happens. In the photo you can see it's a Yuasa 3000 battery. Just in front of the '3' is a tap into the positive lead where a couple of spade terminals derive their supply for the relays. During the course of removing the battery for safety reasons, I'd dislodged this connection. Took 5 seconds to fix but 3 days to find - after I'd eliminated everything else first.
Yeah, I'm just really miffed that it cost me my MOT slot, though.
Whilst I was on my wild goose chase looking for the culprit - and before I realised it was myself - I discovered that the electronic ignition unit, the AB14 from Lucas, is a seriously unreliable piece of junk (just imagine that - Lucas manufacturing unreliable junk!) so I'm thinking of replacing it just to be on the safe side. But what with? Is there a recommended upgrade that you happen to know of?