guarantee on subcontractors' work?

I would really appreciate some advice....we are doing a house extension and remodel including rendering the house in K-Rend. Our main contractor, with whom we have a fixed price JCT contract, has just asked that we pay the renderers they have subcontracted directly. He claims we benefit in this way (a) because they are not VAT-registered, and (b) because we will have the guarantee on the work written directly to us (because it will be our name on the invoice). I am suspicious of both claims but particularly of (b). Surely we will get the guarantee on the work (by guarantee I mean the K-Rend guarantee having been done by registered installers) regardless of whether we pay the main contractor or the renderers?
Very many thanks for any input on this....

As you have a fixed price contract do not deviate from same.Anybody not registered for Vat must be doing very little business.

Avoid!

First of all, you've aready paid for it. Secondly, as Clonback points out, if they're doing so little business as not to be VAT registered, chances are they won't be around if you ever need to pursue a warranty claim.

Thanks - your replies have reinforced the general feeling of alarm bells...