Check the tube that connects the fuel filler to the fuel tank which starts to split as it ages but isn't immediately obvious. Not sure how old your car is, mine is 2004, but it's a known problem where the tube splits and the first you know about it is the smell of fuel and if you leave it long enough, like I did, the next indication is when you fill up and fuel starts pouring out over the exhaust when you fill up with petrol. It's not noticeable when the tank is half full but fill it up and you get the petrol smell that you describe.
