Place the vegetable oil into a deep, heavy-bottomed saucepan and heat until a small cube of bread sizzles and turns golden when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Place the self-raising flour into a large bowl, add the vinegar and egg and gradually whisk in enough sparkling water to make a thick batter. Leave to rest for ten minutes.
Dust the turbot fillet with the plain flour and shake off any excess. Dip the turbot into the batter, then carefully place into the hot oil and deep fry for 4-5 minutes, or until golden-brown on all sides and completely cooked through. Remove from the oil with a slotted spoon and drain on kitchen paper.
Add the potato chips to the hot oil and deep fry for 3-4 minutes, or until beginning to turn golden-brown. Remove from the oil with a slotted spoon and drain on kitchen paper. Return the chips to the hot oil and deep fry for a further 2-3 minutes, or until golden-brown and crisp. Remove from the oil with a slotted spoon and drain on kitchen paper.
To serve, place the turbot onto a serving plate with the chips and serve with tomato ketchup and lemon wedges.