Hello Jo Anne,
Without access to all the facts, no-one can possibly advise you and you should be very careful about your next step. You mention a "devis", and this will be the first key to the whole matter. Without compromising your situation - and bear in mind that your builder might well watch this Forum on SFN, a properly constituted devis is a legally binding contract and there are hard and fast rules about this. The fact that your builder is English has nothing whatsoever to do with it, assuming he is a properly registered artisan or business, and any 'official' letter MUST be written in French - both ways - because it is French Law that applies.
My LeLingo.fr bilingual helpline could certainly help and advise you on this and probably find a mutually agreed solution, but because of the importance of doing the right thing, and of confidentiality, this forum is definitely not the place to air all the facts and issues.
If you would like to contact me privately, you will find my contact details on my personal page, of you could my website and follow the links to the contact page. I would be please to advise and to help.