1. In a large bowl mix the all but two tablespoons of the flour with the 2 ounces of cornstarch and 1 teaspoon baking powder. Season lightly with a pinch of salt and pepper.
2. Using a fork, and whisking continuously, add the beer and the water to the flour mixture until you have a thick, smooth batter. Place the batter in the fridge as 30 minutes to an hour.
3. Cut the potatoes into 1-centimeter slices, then slice these into 1-centimeter-wide chips. Place the chips into a colander and rinse under cold running water.
4. Place the washed chips into a pan of cold water, bring to boil and simmer for 3 to 4 minutes.
5. Drain and dry with kitchen paper. Keep in the fridge covered with kitchen paper until needed.
6. Lay the fish fillets on a sheet of kitchen paper Season a little sea salt.
7. Heat the oil to 250 F (121 C) in a deep-fat fryer. Blanch the chips a few handfuls in the fat for a couple of minutes. Do not brown them. Once the chips are slightly cooked remove them from the fat. Keep to one side.
8. Place the 2 tablespoons of flour reserved from the batter mix into a shallow bowl. Toss each fish fillet in the flour and shake off any excess.
Dip into the batter: