Put the oil in a deep-fat fryer or large, deep-sided saucepan and heat slowly until the oil reaches 180°C. If you aren’t using a deepfat fryer and don’t have an instant-read thermometer carefully add a spoonful of the batter (see below) to the pan – if it floats and turns golden brown within 60 seconds then the oil is ready. If the batter burns then the oil is too hot and should be carefully removed from the heat to cool down.
While the oil is heating, combine the flour, baking powder and salt in a large mixing bowl, then slowly pour in the water, whisking continuously to a smooth batter as you do.
Pat the fish fillets dry on kitchen paper, then working with a single fillet at a time, dip the fish in the batter until fully coated and carefully transfer to the hot oil. Cook the fish in 2 batches for 3–5 minutes each, until the batter is crisp and golden and the fish is cooked through. Remove the fish from the oil and leave to drain on kitchen paper while you cook the second batch. The fish is now ready to be served with chip-shop chips and mushy peas alongside.