Vito dualiner seat conversion

[gavthirdphase]

I've just got myself a vito compact panel van and I was planning to add seats to the rear. Can anyone tell me what the deal is with the insurance if I purchase the floor boards with rails and seats second hand and try and fit them myself? A few company's out there offer dualiner conversion but they are expensive and don't use merc parts. When I spoke to a company that does conversions they claimed that I need seats that are crash test certified. But if I'm using merc parts surely they would be certified. My question really is does the work have to be done by someone certified In order for the insurance company to accept it. Could I get my local mechanic to do it for me with the used parts that I would supply him?