In the North we prefer haddock or whiting, in the South it's cod, but hake, pollack or basa make reasonable substitutes. The secret is a good batter, as I had for last night's tea.
3 Tbs of plain flour (good for four fillets depending on size)
Very cold water, about a mugful
1 dsp vinegar
1½ dsp of baking powder or ¾ dsp of bicarb
Seasoned flour for dusting.
Slowly drizzle the water and vinegar into the flour whilst continuously beating to make a thickish batter. If you draw a figure of eight in the batter the start should just disappear as you get to the end. Set aside for ten mins or as your oil heats up - a drop of batter in the oil should float back to the top straight away.
Now add the baking powder/bicarb to the batter and give a vigorous whisk. Dust a fillet in the seasoned flour then coat in the batter and put straight in the oil. Cook fillets till crisp and golden brown.
* Beer or lager can be used instead of water.