TFT Nick, I suspect that it's going to be a dodgy connection, or I do now I have been told the alternator seems to be OK, or it's an intermittent fault. I'm certainly going to have a good go with emery paper before going back to Woodautos and spending £240. They did say that they would knock off the £15 + VAT test fee though.
Bill, It's a Marchal, so I think you may have something, that and the made in France I just found. The light was on but dim, but it was dim when the engine was off! I didn't notice if it was brighter or not when the revs went up, but it's something I can look for now I know to look. It would be nice to think I could source a suitable replacement in a scrapyard, I'll keep that as an alternative last resort, well, before buying another new one resort.
Ash is going to be putting weight on!
The bigger connection bottom right is marked B+ and takes the big wire.
The other connection at the bottom left is labelled D-
Connection 61 takes the other wire, it's only a spade connection.
The finned jobbie has 3 wires, red to 61, green labelled DF and the black one is connected to the case. Would that be the regulator?
I don't know about "plunging blindly on", you knew more about it than I did and it's sat here infront of me where I can see it.
I have much more idea what to look for and what to try to do a fault tree now.
Edit, Dave, is the "alternator frame" the metal body of it? Where the black wire goes to? I think I undestand the rest of what you suggest to test, it seems logical at any rate.