Trim, halve and bone the plaice (or ask your fishmonger to prepare it for you).
For the batter, mix the flour and cornflour together in a large mixing bowl. Season, to taste, with salt and freshly ground black pepper. Gradually whisk in enough of the water to give you a batter with the consistency of double cream (you may not need to use all the water).
Heat the oil in a deep, heavy-based frying pan until the surface of the oil begins to shimmer. To test if it's hot enough, dribble some of the batter into the hot oil. It should sizzle and brown. (NB: hot oil can be very dangerous. Don't leave unattended.)
When the oil is hot, give the batter a final stir. Dip the plaice into the mixture, then drop the fish into the pan of hot oil. The fish will take 3-4 minutes to crisp up, depending on size.
When the fish is crisp and golden, carefully remove the fish with a slotted spoon and drain on kitchen paper.
Serve hot, with wedges of lemon.