Just before reaching home the other night the battery warning light came on and the a voltmeter nosedived. In the light of day I could see that the belt had come off the pulley on the alternator. It has never happened before. The belt is not worn, the pivot bolt was tight and I had to move the alternator before being able to replace the belt. A freak do you think? extremely hot here.

Possibly stretched a bit in the heat?

Thanks Graham,I'm hoping that that is the case. I don't like weird illogical occurrences.

Are the pulleys all OK?

Think so, could not wiggle them.