in the interests of balance, I had what I would consider to be my best ever customer service experience with an airline, with Monarch only this year.
On a flight back from Menorca, I spotted, whilst loaded my childs car seat in the car, that the isofix attacher click was bent and therefore wouldnt slot into the car. As I only noticed this at the car park, and the seat would also secure via a seatbelt, I thought it best to report it when I got home.
My wife made the call and after just one phone call explaining the issue, the model number and the damage they accepted the claim as genuine, and within about 3 days I had a brand new car seat delivered to the house.
It doesnt help the situation with you in Grenoble but just wanted to report that (from time to time), they can get things very right.