For the scampi, heat a deep fat fryer to 190C/375F. (CAUTION: Hot oil can be dangerous. Do not leave unattended.)
Meanwhile, mix the flour and cornflour in a bowl until well combined.
Pour the tonic water into the flours and whisk in the vodka until the batter is about the consistency of double cream.
Coat the langoustines in the batter and fry in the oil for 3-4 minutes, or until golden-brown and crisp.
Remove the scampi with a slotted spoon and drain on kitchen paper.
For the mayonnaise, place the eggs, white wine vinegar and mustard into a food processor and blend until pale and creamy.
With the motor running, pour in the oil, in a slow steady stream, until the mayonnaise is thick (you may not need all the oil). Mix in the lemon juice.
To serve, pile the scampi in a serving bowl and serve the lemon mayonnaise alongside. Finish with a sprinkle of sea salt and a squeeze of lemon juice.