You can download Westfalia's instructions from their website and it shows how to do it.
FWIW, I bought an Invisihitch. They're based in SA, but also have a factory in the USA, but I'm sure they market worldwide. It isn't quite as slick as the Westfalia's auto-retracting hitch, but it still becomes invisible if you pop out the receiver. Not sure if they have a UK website, but the USA one is www.execuhitch.com