An update on my last post having finally managed to speak to a very helpful chap in the DGAC in Paris.
What he told me is that it is illegal without an AOC to offer air transport services (ie an aircraft and pilot as one service). However, it is legal for third parties to hire an aeroplane without a pilot, and then hire a pilot separately to fly the plane (as long as he is qualifed to fly the plane).
Thus the flights that I have undertaken in the past were legal as the hire of the aircraft was invoiced directly to either my company, or the yacht in question. I did not actually invoice for my time as a professional pilot, though I could choose to raise a second invoice if I wished.
So voilą, in France at least it is legal.