Bring a large pot of water to the boil. Nip the tail of the langoustine and twist it, pulling out the black vein that lies down the middle. Alternatively, remove vein after cooking.
Drop the cleaned langoustine into the boiling water and boil them for 2-3 minutes, just until they change colour.
Set aside to cool. (They’re fabulous to eat just like this dipped in mayonnaise, never mind frying them).
Shell them by squeezing the shell with your fingers, cracking it along the back. Turn the langoustine over and use the length of both your thumbs against the edges of the shell on the underside to pull the shell apart. The beautiful pink flesh comes out in one piece. Check the back to make sure the entire black vein is out. If not, rinse it away with a little water.
Mix the egg and the cold water and sift the flours into the bowl. Whisk everything with a fork or balloon whisk. The batter should be thin, just enough to coat the seafood lightly. If it’s too thick add a little more water.
Heat oil for deep-frying. Dip the langoustine into the batter, shake away any excess and deep-fry them until they are light and golden. They’ll cook in 3-4 minutes. Keep the oil nice and hot.