Heat the oil in a temperature-controlled deep-fat fryer to 120°C/250°F, or in a heavy-based saucepan, checking the temperature with a thermometer, to blanch the chips. Preheat the over to 150°C/300°F/Gas Mark 2.
Fry the chips for about 8-10 minutes, depending on size, until soft but not coloured. Remove from the oil, drain on kitchen paper and place in a warm dish in the preheated oven. Increase the temperature of the oil to 180°C/350°F, or until a cube of bread browns in 30 seconds. Meanwhile, season the fish with salt and pepper and dust it lightly with a little flour.
Make a thick batter by sifting the flour in a bowl and whisking in most of the lager. Check the consistency before adding the remainder: it should be very thick like double cream.
Dip one fillet into the batter and allow batter to coat it thickly. Carefully place the fish in the hot oil, then repeat with the other fillet.
Cook for 8-10 minutes, depending on the thickness of the fish. Turn the fillets over halfway through the cooking time. Remove the fish from the fryer, drain and keep warm. Make sure the oil temperature is still at 180°C/350°F and return the chips to the fryer. Cook for a further 2-3 minutes until golden brown and crispy. Drain, season and service immediately with the battered fish and Tartare Sauce. Garnish with the parsley and lemon wedges.