fuel coming from filler cap

hi there can any body shed some light on a problem i have with a1998 r reg 3oo tdi station wagon .if i fill it with more than a1/4 of tank full of diesel all it wants to do is blow it out the breather on the fuel cap other than that the 110 drives fine with no other faults .this is a real pain and spoiling a nice vehical .

There's a smaller tube running from the top of the tank to the filler neck, is this blocked.

hi no that tube is clear i can blow throught it quite easy and hear it in the tank

Is this fillercap a refit with a low pressure pressure cap? We took a 205gti for testing at silverstone the other week but someone had refitted the coolant cap (pressure cap) with one that fitted in the ole. but it was lower pressure than the system so pished coolant everywhere!