I had the same problem with a 1961 Skoda I found in a garage, which had slipped off the D.V.L.A.'s records and had since been re-registered with a non date-specific registration number. The Skoda Owners' Club were magnificent in assisting me, as they-and I suspect most classic car clubs-have a member who can authenticate the date of manufacture and this is taken as correct my the D.V.L.A. As a result, in my case, the D.V.L.A. re-issued the original registration number (I had the original plate in there boot!) and altered the date of first registration to 1961, as opposed to 1984, which it had been.
If you're not already a member of the Renault Club I would suggest joining and asking for their help-I'm sure it's well worth the fee!