diesel leak from cap

Hi, guys,
I'm new to owning a freelander 1, its a 2ltr TD , I'm trying to find out should there be a little metal flap in the neck of the filling point?
as I don't have one and was wondering if this could be why when cornering diesel seams to leak from the cap.
if anyone has any ideas as to why it is leaking it would really help me out.

You really should introduce yourself before posting - Introduce Yourself - Land Rover Zone

Yes there should be a spring loaded metal disc in the filler hole , Mine fell into the tank many years ago
It never leaked diesel even when full up so would look at replacing the cap ?
http://www.ebay.co.uk/itm/Land-Rove...CarParts_SM&hash=item43cff4aebf#ht_551wt_1031

Hi, sorry for not introducing myself. but big thanks for the help.