It would be wise to check now that this was the insurer the builder had at the time the work was completed. He may have changed insurer since, but I believe the run-off of responsibility would remain with the original insurer, not be something picked up by his new one.
There is an interesting article on assurance décennale (well, interesting to those of us from the world of insurance, at least!) on French wikipedia : http://fr.wikipedia.org