One old trick that I've seen was to fit another windscreen washer nozzle on the bonnet and run the breather pipe to that. The hole is too small for a rain drop to go down.

Thanks Paul & Doon - again, much appreciated 

I started looked at one way valves and the price variance is crazy as I've no doubt the quality is too!

https://www.demon-tweeks.com/uk/mocal-in-line-fuel-tank-breather-valve-23340/ for near as damn-it £50 to £5 on Amazon...

I like the idea of a small hole under the latch, that could be the way forward, cheers guys!

Yes you would need to pay the Demon Tweeks price......  The £5 .....  speaks for itself.   Mine came from Car Builder Solutions several years ago.

https://www.carbuilder.com/uk/in-line-fuel-tank-breather-valve-jic-6-male-unions

Neil    ....  Don`t overthink this    ...  the solution is simple... All you need is to keep the fuel tank at the same pressure inside and outside. As Doon says a simple hole will do the job. You don`t need to install a one way / roll over valve. A Beetle never had one. In my case it was a "belts and braces thing for IVA.....

They didn't need a valve as the route of the breather pipe would put the inlet/outlet above the tank if (and when) they rolled.

I like the idea of a small hole under the latch, that could be the way forward, cheers guys!

Strongly advise against that one as when the fuel is used and you get negative pressure in the tank, it can actually draw any moisture or water through the hole   

Good point but surely only if the hole is at a low point or it's really chucking it down. Depends on the orientation of the filler as well if it's at the centre of the bonnet and flat then risky as it could get a puddle but would it work if it's more of an angle? How is it really different to a vented cap and hole can be miniscule

Fair point, but if drilling a hole through a lump of ally, to save drill breakage its going to be a few mm in size and not miniscule like the OEM vented ones