Buying from a French Car Dealer - Negotiation?

I've been here five+ years now and I've noticed that (in my limited experience) the French don't negotiate as readily as in Blighty. Sometimes they look affronted and are very reluctant to discount, but sometimes they do.

My question is, I'm looking to spend quite a bit on a car from a dealer soon and I'm not sure how hard to go in on my first offer against their asking price? In the UK I'd reckon on starting about 10% below asking, expecting to get 5% without any part exchange. But here??

Interested in any thoughts or similar experiences...