Bring a large saucepan of lightly salted water to the boil over high heat. Add chips and return to the boil. Reduce heat to medium and simmer for 5 minutes or until just tender. Drain well and spread on paper towel to dry completely.
Meanwhile, whisk flour, lager and 2 teaspoons salt in a bowl until smooth. Cover with plastic wrap and set aside at room temperature for 30 minutes.
Half-fill a deep-fryer or large heavybased saucepan with oil and heat to 160°C. Fry the chips, in batches, for 5-7 minutes until soft but still pale, then drain. Chips can be prepared to this stage ahead of time. Lay flat on a paper towel.
Preheat oven to low. Increase oil to 190°C. When hot, fry chips again, in batches, for a further 2-3 minutes until crisp and golden. Drain on paper towel, place on a tray, in a single layer, and keep warm in the oven while you cook fish.
Dust flathead fillets in the flour. Dip into the batter, one at a time, allowing excess to drain off. Carefully place into the hot oil. Cook the fish, in batches if necessary, for 3-4 minutes until golden and crisp, then drain on paper towel.
Remove chips from oven and sprinkle fish and chips with a little more sea salt. Serve immediately with lemon wedges and iceberg lettuce.
4 large pontiac potatoes, peeled, cut lengthways into 1cm-thick chips
185g plain flour, plus extra to dust
330ml lager Sunflower oil, to deep-fry
800g skinless flathead fillets, halved into long strips if necessary