Update.
The bracket is also knackered and i am getting one from Beta.
I think it best to replace the aged alternator as well, instead of trying to repair. It is difficult to source though ( yes, an old ISKRA) and despite cross referencing, i may have to get it from Beta, who in fairness have been helpful. It's a bit more than expected, but not extortionate, and i know it will be a perfect fit.
Beta offer a 120A upgrade, and all things considered ( pulleys, bolts, brackets) it is about £150 more. Is this a worthwhile option and will life be better? We are not liveaboards, generally long weekend boaters, apart from 2 x 2 week hols, and have fairly modest power needs, 12v fridge, LED lights, have a Sterling Combi Quasi 3kw inverter charger for shore, microwave, and hairdryer needs (don't ask) , and 4 newish 100A leisure batteries. Separate starter Alternator.
Sorted the main fuse BTW - is was a 150A one blown. Still embarrassed ....