Fuel leak from around filler cap - P38 BMW DSE

This seems strange! Anyone else had this?

After I fill the car with diesel and drive for a while, there are marks on the rear q-panel around the fuel filler flap where fuel has spilled out. It looks as if fuel was spilt when refuelling - which it wasn't!!! In relation to the "air release" on the cap, when turned from fully closed, it makes the usual hissing sound where the air is being released. This suggests that the tank and cap is airtight. Could the fix for this be as simple as a new filler cap ... or might it be something more? It only happens when the tank is full, suggesting that its something to do with fuel splasing near the filler cap/nozzle

Any suggestions for a cure would be most welcome

Thanks

Jonny

Exactly the same issue with mine.

Think it has probably got more to do with the tank being overfilled than a problem with the system. In theory you are supposed to stop filling as soon as the filler clicks to a stop - but most of us give it that extra little bit until it is right to the top!

Matt.