Hi, my top radiator cap has a started to leak under pressure
Any ideas as why would the cap suddenly fail

It happens to radiator caps. They can fail or wear out. There are moving parts inside them (a spring?) designed to allow coolant to escape if the pressure gets too high due to high temperature. If your coolant temperature is normal, try a new cap. If it still leaks, you have other things to check.

its not leaking from the valve, its leaking from under the cap if thats makes sence

It could just be the rubber gasket that's perished or compressed - Radiator caps are cheap enough, so just get a new one as the internal springs etc. will be worn too. Replaced one on a friends NC29 that was always overheating and blowing water out the coolant bottle - runs perfectly now for not a lot of money...