Yes, found it; it looks to have 33310 Lucas No. on it. (Thought it was an early O/d one?) Can't vouch 100% that it is OK, but if you PM me can sort out a nominal cost for it. Cheers. G
Good tip there. I have a s/h 6RA (4-pin?) somewhere, but it may have the same issues being of the same vintage...albeit it has not been on a car for 40 years! If I can find it I will let you know. Cheers. G
Just in case anyone has the same wildly swinging ammeter needle I had, here's what I found. Shorting C1 and C2 on the 6RA field isolating relay gave a steady needle and steady voltage too. It was a bad contact within the relay and no amount of cleaning or soldering joints would cure it. I've fitted a modern relay until a new Lucas 6RA can be sourced. Good luck!