Cut the potatoes into evenly sized chips, about 1cm (½in) across. To remove excess starch, put them into a bowl of cold water and swish around. Dry in a clean tea towel or with kitchen paper. Heat up the oil in a very large, deep saucepan and use a kitchen thermometer to check when it has reached the desired cooking temperature of 130°C (or heat your deep fat fryer to this setting).
Carefully add half the chips to the oil and fry for 7 minutes, until pale and soft. Stir them a few times as they cook, very carefully, to stop them sticking together. Lift from the oil, drain on kitchen paper and allow to cool. Let the oil come back up to temperature, then repeat with the second batch.
When the chips have cooled, preheat the oven to gas 4, 180°C, fan 160°C and line a baking tray with nonstick baking paper. Heat the oil in the pan or fryer to 180°C. Add half the chips and cook for 3-4 minutes, until crisp and golden. Drain, put onto the paper-lined tray in a single layer, scatter with salt and put into the oven to keep warm. Repeat with the second batch.
To make the batter, sift the self-raising flour into a large bowl and add a generous pinch of salt, then pour in the beer. Whisk together to make a smooth, thick batter.
Heat the oil in the pan or fryer to 180°C. Pat the fish dry with kitchen paper, then dip each fillet into the cornflour. Shake off the excess. Dunk two fillets into the batter, one at a time, then carefully place in the hot oil and cook until golden and crisp (about 5 minutes). Lift the fillets out of the oil, drain on kitchen paper and keep warm in the oven with the chips. Let the oil come back up to temperature, then coat and fry the remaining two fillets. Serve the battered fish with the double-cooked chips, lemon wedges and garden peas or mushy peas,
if you like.