You can access the combined pump/sender without removing the tank (TD5 assumed!) the RAVE manual shows removing the 3rd row seats (if fitted) then O/S rear side trim which allows the carpet to be pulled back to access the removable panel in the floor to gain access to the unit.
Check your wiring connections first, you never know, if a wire is rubing against metal and allowing an earth path, this could alter the reading although its unlikely. Chances are either the gauge or sender are faulty. With older vehicles you could test the fuel gauge by removing the wire from the sender and touching it against a good earth point, if the gauge rises to full, its working fine and the sender is faulty, not sure about trying this on a Disco with more complex electronics.