Spacers should not be necessary and as Otherclive has suggested, they should only be used if they form part of the type approval for the vehicle.
I just need to clarify two points related to Woodentop's comment.
...The ball horizontal centre line should be between 350mm and 420mm - or in old money 13¾-16½ inches - off the ground when level.
The regulation that the loaded towball must settle at between 350 and and 420, does not apply to all cars, some cars are exempt and it is possible the Disco may be one of them as it may be classed as a special purpose vehicle. Check with your vehicle supplier.
The second point is there is no requirement for the trailer or the car to be level when loaded. In fact it is highly unlikely any car or its trailer will be level when loaded.
However the height measurement should be made when the coupled outfit is on level (horizontal) surface. perhaps that’s what WT meant.