Hi Chris, sorry I didn't saw your first message. Well, as Anna said this is not really a claim you make to the Protection Juridique but rather you seeking advise with them. It's clearly a good idea and can set you up on tracks.
But it depends how you want to play it, if you want to play it hard ball from the start then you can send the registered letter upfront, note that the legal protection usually take some time to process your requests so I would highly advise you to call on them ASAP to start the procedure. Alternatively you can of course try to reach out for the builders stating clearly that you'll file a complaint to their 'Décenale' if they don't fix it or start a legal procedure against them (personally) if they were not covered.
To be honest if you've already done some reparation work that doesn't smell good as it will be difficult to prouve the prejudice.
Anyway I hope this is helpful,