Many thanks for all the info folks. Vehicle electrics are a mystery to me. I was hoping there'd be an off the shelf one with the same wiring setup, but I guess I can study the wiring diagram - although it might as well be Hieroglyphics!
I'd been struggling on with the previous relay as I've not been using the bike much - sometimes it worked and sometimes it didn't. I kept blaming the battery, but I've had it on trickle charge for a while so it's all charged up. I was mostly using the kickstart, but the other evening, when I was really in the mood for a ride on the Enfield, it just would not start! By the time I'd kicked and cursed enough it was getting dark and the mood had passed. So yesterday I unwrapped the new relay.
It began playing up as soon as it was fitted. It did the old trick of first nothing, then continuously spinning the motor even with the ignition switched off, until whacked with a large screwdriver handle. It's worked ok a few times since, but a bit hesitant.
Not to be trusted anyway.
There was a rumour that RE were now fitting decent Jap made relays - but this latest one certainly looks like all the others I've had. I might phone Watsonian and ask whether they've got anything better.
Starting to think this bike's been cursed by the gypsies......