Hello again, Sue.
Here (Brittany) there are many British folk, and regrettably some that don't / won't speak or learn French, making them easy prey to the cowboys I warned of.
As I understand it, and a legal view would probably be welcome; if two parties enter into a contract the aforementioned contract is treated as being subject to the laws (and enforcement) available in the sovereign territory in which the contract is signed (regardless of language used); and to which it relates, UNLESS both parties agree that the contract is subject to the laws of a.n.other country. And even then, there needs to be some plausible reason for so doing, such as parent company ownership from outside the EU.
In your circumstances, my expectation would be that as you are domiciled in France (yes?) and would be contracting with a company also domiciled in France the contract would be subject to French laws..... which is probably preferable to suing a builder remotely through the British legal system - the value of the roof would be eaten up in costs associated with 'case conferences' 'pre-trial hearings' etc etc. If the contract was worth many millions of euros, then it might be worth considering, but the French system, whilst a little arcane to the outsider, is sound and you can rely on it to protect you from fraudsters and conmen (and women).
You will be checking their SIREN / SIRET code and confirming their APE (Activite Principale Exercee) number too...(start with www.insee.fr )
Looking at Sarah Crosby's reply (below) her employer seems to have the right approach, and if they are by any chance located in Brittany, I'd be pleased to receive their details!
As others have mentioned, the insurance against latent defects / defective workmanship which may take years to become apparent, is of questionable value. I have a case at present where the roofer is trying to wriggle-out of responsibility by reference to the recent (agreed "exceptionally") heavy rain we have had and is willfully denying that the function of a roof is to keep water out....... (I'll name no names...unless anyone in Brittany wants to contact me directly).
In conclusion, choose your contractor with care, but start from a point of mutual trust; don't expect to have to sue them, but make it clear, with a steely look in your eye, that you will....if it becomes necessary; privately, hope that it doesn't.
John