Neil, in order to narrow things down a little what hitch does your van have. I'm guessing it is either a basic Alko, Stabiliser type Alko or possibly a big blue Winterhoff. A photo of the hitch and a couple of the bellows would help. I do have a new bellows but I'm not sure of the number/fit as I'm away at present.
If you could post some photos I should be able to check for you Sunday/Monday when back home.
Colin