Thanks to everyone for their input.
Everything checks out, no unusual noises, pullies running true etc - just the damaged alternator belt.
This time it stopped in a more revealing place - it looks like something has stripped one edge off the belt and there is some damage to one of the internal ribs, the pullies look fine.
I'm working on the 'foreign object' theory, although what and how it could have flicked up is a mystery.
About to order new cambelt kit (due anyway) and alternator belt, should solve the problem.