Huh! Here's a thing.
I wanted to make sure about my thinking, so I had another look today. Sure enough, 3.3 amps was pulled when an ammeter was put across the relay terminals. 3.8 amps when on charge.
I disconnected the clutch plug on the compressor, and checked again. Zero amps. So it seemed fairly certain that the current was flowing through the solenoid coil.
It was about now, with the power on again, that I hit the clutch with a big hammer. It clunked on.
Big hammers were a standard tool in post-war Brittan
I found that it didn't spring out again, so I prized it out, and hit it again. Several time. Now it works by itself.
The thing is that I would like to dry-lube the bit that slides, but daren't do this if I'm going to get graphite on the clutch surfaces.
Anyone have an exploded diagram of the inside of the clutch?