Leaking fuel and loads of back pressure from tank? HELP!

My friend has a problem with her Golf but we're not sure of the cause- the car leaks petrol if it is filled it over half full. Also when you take the petrol cap off there is loads of pressure behind it and if you dont unscrew it carefully then petrol spills out. Leak suggets it is a rusted filler pipe problem but what is the back pressure? Blocked return fuel pipe? Car is a GTI.

You need to look in Haynes manual at fuel breather section .

Pressure [slight is normal] to the tank is normal , it helps the pump keep fuel to the engine . But look in the drivers wheel arch you will see a 'Gravity valve'
this must be the correct way up and free from obstructions , it also keeps the tank from pressurizing and causing the problems you have .

Thanks, we'll have a look at that. We'll also check the filler pipe in case it rusted.

I have the same problem!