In the classic English version, whisk together 115 g/ 4 oz sifted plain flour, 1 egg, 150 ml/ 5 fl oz milk and 1 tsp salt, then leave it to stand for 30 minutes. It should be smooth and creamy.
For a tasty light beer batter: sift 115 g/ 4 oz plain flour, add 1 tbsp olive oil, then whisk 200 ml/ 7 fl oz cold beer into 125 g/ 4 oz plain flour. Leave to stand for 1 hour. Whisk the whites of 2 eggs to soft peaks, then fold them in just before coating the fish fillets.
For a substantial thick batter that holds up well if the food is not served immediately: sift 225 g/ 8 oz plain flour into a bowl and add 4 g(⅛ oz, ½ sachet) casy-blend yeast, 1 tsp salt and 300 ml/ ½ pt hand-warm water. Whisk together. Cover and leave to stand at room temperature for 90 minutes. Stir before use.
For one of the lightest and crispest of batters: sift 200 g/ 7 oz self-raising flour with 1 tsp salt and ½ tsp baking powder into a bowl and whisk in 300 ml/ ½ pt cold water to a smooth batter. Leave to stand for 1 hour. Whisk again before use.
For a crisp batter: whisk an egg in a bowl with 450 ml/ ¾ pt of milk at room temperature, a sachet (8 g/ ¼ oz) of easy-blend yeast and ¼ tsp caster sugar. Sift 175 g/ 6 oz plain flour and 115 g/ 4 oz cornflour into another bowl with ½ tsp salt. Whisk the milk mixture into this to make a thick batter. Cling-wrap the top and leave for 2 hours when it will have increased in volume by about a half.