To make the batter, mix the yeast, salt and sugar in a bowl until well combined. Pour the tonic water into the yeast mixture and whisk in the flour, gin or vodka and vinegar. Leave the mixture to ferment – it is ready to use when the mixture starts to bubble.
Coat the cod/haddock fillets in the batter and fry in the oil for 3-4 minutes, or until golden brown and crisp. Remove the battered cod with a slotted spoon and drain on kitchen paper.
For the tartare sauce: whisk together the egg yolks and vinegar in a bowl, or a small food processor. Still whisking, start adding the vegetable oil very very slowly to make a mayonnaise. The mixture should be thick and light once the oil is incorporated.
Stir in the capers, gherkins, shallot and herbs. Season with salt and pepper and sharpen with lemon juice and place into serving pots.
Serve with chips and if you're like me you can't have fish & chips without curry sauce.