That's the 'exciter wire' that tell the alternator to charge, it should connect to the battery light circuit somehow, check other cars wiring diagram, there may be an easier solution than changing another alternator.
Thanks for quick response. I have connected 'L' to the battery light wire and the light goes out when I short the main alternator ourtput 'B+' and the exciter '+' so the 'L' connection seems correct but '+' wrong.
Where do you suggest I connect the exciter terminal '+' rather than to 'B+'?
P.S> I believe that scrapping the car is easier than changing the alternator!
But just changed cam belt and water pump so seems a shame!