The fanbelt went today... I think it had been slipping a little for a while, as I've seen occasional flickers from the warning lights for a couple of weeks...
After a certain amount of head scratching, I piggybacked a pile of cable ties, to keep the water pump in motion... which worked better than I expected.
I'd no idea where the belt I had came from, but it seems that a normal 2.5N/a belt fits fine, indeed if anything it is a better fit than the previous one... the alternator is no-longer bang next to the chassis rail.
As I type
8 years ago