Just been looking at my rear tyres on a Fab vRS, which are Michelin Primacy, the sidewalls have quite a lot of fine cracks or crazing.

Are they knackered? I guess they're the original tyres (2004, 35k miles).

Personally I'd ditch them, sounds like they are starting to decay. UV light kills rubber, assuming tyres still have some natural rubber in them.

yeah rubber dries and cracks with age

As long as the cracks are only on the surface, it shouldn't make too much of a difference to be honest, although if they're deep enough to weaken the carcass and causing bulging, then you'll have to get rid. The Primacies are a bit pants anyway compared to some better-priced alternatives, so if you've got any doubts about them, I'd replace them on those grounds as much as any other!