I take on board everyone's comments thank you for that. This is obviously a grey area, though this does beg the question - how or why would the DWP produce this letter if they didnt firmly believe that UK citizens as part of the EU could not work here in France legally , the letter does imply a time limit of two years, and the said builder is not a limited company just registered as a sole trader. He is requesting payment in euros and produces a quote using his french address and french banking details, all of which I think he firmly believes he is legal.