I noticed on this warm day that there was a smell of petrol from the filler area, is this cap a pressure relief cap that is fitted to the car, as, if it is it shouldn't work, as I have an atmosphere breather now fitted to the car, or is it a plain breather. it did come apart on me the other day and I'm wondering if that's done some sort of damage to it, It def. held quite high pressure b4 I fitted the breather in a hot garage, any thoughts?

 

RGP
It should be a solid cap OEM, such as this:
FUEL / PETROL FILLER CAP JAGUAR XJS - CAC2961 | eBay
If your cap has a release spring that is less strong than your valve, that would probably explain the fumes.
Greg