when i bought mine it just had a twist off filler cap, and i got worried about me fuel too, so i bought one of these ....
LAND ROVER DEFENDER LOCKABLE FUEL FILLER CAP`86 TO`99 on eBay, also, Land Rover, Car Parts, Cars, Parts Vehicles (end time 05-Jul-08 17:10:59 BST)
it fitted and worked...