I'm back from the Val and am beginning to sort out all my pictures, though I might just take a long nap before I finish!
Here are a few relevant ones.
The first is blue spot:
Although the line is slightly broken, it is still very solid and 'painted on' compared with the line in this next picture, which is sloe hairstreak (despite the fact it shows quite a prominent blue spot):
The orange spots in sloe hairstreak form contiguous arches, whereas in both Martin's and my blue spot they are separate, with just one prominent orange spot and the rest black dots with orange. That fits yours, too, Paul.
This is a more typical sloe hairstreak:
Finally, an ilex hairstreak, showing a quite different pattern, with a rather weak line and separated but prominent orange spots (with black on both sides):
So, I agree with Martin, that your spotless hairstreak is blue spot. In the Val d'Aran these were quite fresh, but it is not at all surprising that in the South of France they were all very worn - they will have been on the wing since the beginning of June or even before that
Guy