Not sure how this all works,a friend of mine just bought a French registered car(still on French plates and Carte Grises not modified to new English garage owner).The old owner had already filled in the necessary French papers for change of ownership and that all sorted out ok.
I know that you are doing the inverse but I would think that a bill of sale showing you bought it in France and the French Carte Grises you should be able to get a suitable change of ownership/nationality(of car) from DVLA.
You may need to get the old owner to get a "Certificat de Cession D'un Vehicle" and "Demand de Certificat d"Immatriculation d'un Vehicle" and "Certificat de Situation Administrative Simple".
I think you'll only need the first form if any,but never having done the same procedure as yourself it's hard to say as French red tape is sometimes a minefield.I'm sure that the DVLA will be able to come up with the correct papers.
If you need any more help drop me a line and I'll try and find out what the procedure is:nenau:thumb2