Thank you for the ideas. I had already checked the belt tension and it is fine, but you are right to mention it - no point in getting into the clever stuff if there is an easy solution.
There is 12v on the green wire with the key turned to first position, no voltage on the purple or brown wires.
I've had the alternator apart but it is a bit of a mystery to me, though I did find a high reistance between one of the brushes and the spade terminal where the green wire is normally attached, so I bypassed that. It still does not work.
The lettering stamped on the side of the alternator seems to read 1103116 61a 1k9 12v NEG
The green wire leads to the brush nearest the front (pulley end) of the alternator
The brown wire leads to te brush nearest the back of the alternator
The purple wire leads to the top of one of the three diodes which can be seen when the casing is split
I thought six diode were needed, not three - am I wrong ?
There is a capacitor inside the alternator, and also a small black box about 2 cm x 1 cm
I could no test the diodes without further dismantling the alternator
Any further ideas, please ?