Hey mate. does it have to be adjustable? If not you can buy off the shelf a replacement hitch at different heights. I went to the local Hayman Reece dealer in Shepp and they had 4 or 5 different height hitches as well as the adjustable type. Its just a matter of measuring what height you want. I wanted 40mm higher but even the adjustable one couldn't get 40mm (it went higher than that). I couldn't go more than 40mm because of the tailgate on the use when it folds down.
If there is a local Hayman Reece dealer to you call in and see what they can do.