Studying the photographs I took at the time and looking through literature reveals that many of these birds look like hybrids. Clearly there are pure (at least pure-looking) Fischer's Lovebirds there but they appear to be outnumbered by hybrids which begs the question whether these feral populations are really viable.
Interestingly, the hybrid birds look like Fischer's x Hooded (see the copy of the plate out of the Helm Guide to Parrots of the World below) which are also known to hybridise in Africa. On my visit to France however I didn't manage to photograph any pure looking Hooded Lovebirds although there were some present.
Some of my photographs are shown below. Not great quality as it was in my old digi-scoping days!
Anyone who has been to see these birds or is contemplating a trip should bear this in mind and (while they remain on the WP list) make sure they see the right birds!
Plate from Parrots of the World