To prepare the langoustines, pull off the head and pincers. Lay the tail flat on a chopping board and use a sharp pair of scissors to cut a line straight down the back of the shell. Carefully peel the langoustine, score down the back, then remove the grit sac.
Get the oil heating in a large saucepan or wok – you will need enough to come 2-3in up the side of the pan. For the tartare sauce, mix all ingredients in a bowl and season.
Place the flours in a bowl with a good pinch of salt and pepper. Add the beer and sparkling water, and whisk to a smooth batter.
To test if the oil is hot enough, put a drop of batter into the pan – it should crisp and brown within 30 secs. Dip each langoustine or prawn tail into the batter, then carefully drop it into the oil. Drizzle a little extra batter over each one while they are cooking – this will give you a really crispy coating. Cook them in batches, making sure you don’t overcrowd the pan. When golden and floating to the surface, scoop out and drain well on kitchen paper. Sprinkle the scampi with salt and serve with the tartare sauce, lemon wedges and chips.