Noticed while out and about tonight that the back left corner of the car reeked of petrol, got under it and can smell it again.

Nothing on the ground, think it's a vapour leak -do these tanks have a pressure relief valve? After all it was nearly 35C out there today so I wouldn't be surprised if there was an excess of pressure in the tank!

Any common points to look out for?

Check the hoses leading to the tank as they could leak. like all fuels they can expand with heat. Also if the smells still there check around the sender unit for fuel leaking either perished seal or worn hoses.