Fuel Smell

I've had a bad fuel smell outside my 2A for a while now, and have had issues tracing it. No leaks in the tank, and there's no smell inside... even though the rubber hose is very perished.

This leads me to think it's evaporating petrol from the tank via the filler cap. So the question is, can I get a new rubber seal or do I have to buy a new complete cap?

It's just a large o ring find a tractor shop and get one or the seal if LR or **** part still sell them

Also, there's a ball bearing in there which is supposed to allow vapour to fart out but stop water getting in, it may be stuck...