When my girlfriend drives her car round corners.. fuel spills out of the petrol cap. is there any reason for this? Or does she need to go slower. I looked at the filler neck and the car doesnt have a one way valve... is it meant to?

That shouldn't be happening!! sounds to me like the filler cap is nor sealing properly. not sure how to go about fixing it tbh, either find one in a scrapyard, you should be able to swap the lock barrel over to keep the same key (i think) or buy new one from fiat. I do remember someone doing this not long back and they found there is a few different caps so be careful you don't get a wrong one.

I'd suspect that either the rubber seal on the cap itself is fubared (or missing) or that the piping has come undone somewhere. If it's the seal, you may be able to get it seperately.

In the meantime I'd advise filling the tank less as spilt petrol is as bad for bikers as oil or deisel.

And a quick search brings this up

https://www.fiatforum.com/cinquecento-seicento/67845-petrol-caps-warning.html?highlight=petrol+cap