Thanks for the links rayc. Unfortunately the seller will not ship to NZ and most they can do is send in UK and i'll have to arrange courier from there. I do recall NZ Post having such a service where they will handle the pickup etc but will have to confirm. At the moment i'm trying to find what I can in NZ and explaining about the PCD difference is the key. Specialty mag shops tell me I can just use a different profile tyre to suit but i'm certain not all profiles are cost effective. Two local shops say I could go with the Momo 17" rims with a lower profile tyre but I know it will be an expensive option (Momo rims along $1400NZD so about 700 GBP). Will reply as soon as I get a response from the local Fiat dealer.
I have a local van wrecker that is parting out a newer model Ducato and will get back to me on the rear tyre carriage mechanism. Below are photos under the van. There are 2 cross beans with pre-drilled holes. Some of the carriages sold online differ slightly in terms of mounting.
https://i.imgur.com/yEadyBY.jpg
https://i.imgur.com/PAikN3U.jpg
https://i.imgur.com/zXpEd4q.jpg
https://i.imgur.com/PqM04Ce.jpg