I hope we have cleared this murky subject up. Not sure why it comes up so often !
If you are offered anything that isn't Nephlops Norvegicus, then refuse to pay for it. Langoustines are easily available just about anywhere, and are extremely easy to cook, taking only 5 mins. With a glass of Chablis and plenty of crusty bread, they are a veritable feast.
Best bought between September and May, Whenever I am in my 'van in Cornwall, I go to the fisherman's cooperative in Newlyn, and buy a huge bag for less than a tenner.