To make the tartar sauce, in a small bowl mix together the mayonnaise, parsley, cornichons, capers, malt vinegar, lemon juice, lemon zest, and salt and pepper to taste. Place this aside, until ready to serve. Preheat the oven to 325 F.
Heat a deep fat fryer to 250 F, and once the oil has reached temperature, add the potatoes. Allow them to fry in the oil for 10 minutes, until they are cooked through but not browned. Remove the chips from the fryer, and drain them well in a bowl layered with a paper towel.
Turn the deep fat fryer up to 375 F, and allow it to reach temperature before adding the chips to the oil for a second time. Cook them for 3 to 4 minutes, until they are crisp and golden. Remove them from the fryer, and place them in the oven to keep warm.
In a large bowl, whisk together ¾ cup of flour, baking powder, salt and beer to form a batter.
Coat the fish fillets with the remaining ¼ cup of flour, before coating them thoroughly in the beer batter, and then dropping them into the hot oil heated to 375 F. Fry the fish for 2 to 3 minutes, until the batter is golden, and the fish is cooked though.
Serve the fish and chips straight away with the tartar sauce, a slice of lemon each, and a side of peas, if desired.