It had been a wet winter and although I waited for a couple of dry sunny days before climbing I suspect water had weakened the spike. The route and all nearby cracks were dry, if a little green. Placing a cam near the base and a sling over it must have exerted enough force to cause a break when shock loaded. Lesson to learn - be very wary of gritstone after an extended wet period. It takes longer than a couple of days to dry out.