pop the flap open and bend the metal part of the catch towards the pin, push the flap to and check the gap between the metal part of the catch and the pin, it should be as close as poss without touching.
it's a common problem with these as the metal is very soft and if you knock it with the pump it bends very easily.
Another cure, I've found that the button for the flap is in the wrong place and it's easy to knock when putting the key in the ignition (2003 models onwards). if your vehicle doesn't have cruise control, there is a spare switch blank above the fuel flap button, move the button into the spare hole and put the blank in the hole the fuel button came from, that way you wont accidently knock it