Eurotrash, I don't make this stuff up, and some things fall between the cracks.
I contacted the Inspection de Travail and UK equivalent and in spite of this 'fantastic' EU, neither would get involved because..............
And there was a french set of books in France showing everything above board, and another set for the UK employer.
Was this specific company 'chancing it', only sort of or not really, as the authorities in both countries were not interested. The staff were working for £1 an hour plus lodging and food. And if you think that french staff in say ski ressorts or other holiday places work for that much, think on. And they get conned too, but NEVER to that extent.