Pre-heat the oven to 150°C/130°C Fan/Gas 2. Line two baking trays with double-thickness kitchen roll. Peel the potatoes and cut into thick chips. Put them in a bowl of cold water and set aside until you're ready to cook them.
Boil the peas with the mint leaves for 5 minutes. Drain and put in a bowl. Add the crème fraîche, then blitz with a hand-held blender to get a rough purée. Set aside and keep warm until ready to serve.
Sift the four into a bowl, add a pinch of salt, then gradually whisk in the lager (or sparkling water) and soy sauce to make a smooth batter that is about the thickness of double cream. Pour sunfower oil into a large pan or deep fryer until one-third full. Heat to 120°C, or until a small piece of bread sizzles gently when dropped in the oil.
Drain the chips, then pat dry with paper towels. Put enough in a chip basket to fill a quarter of the way up - don't cook too many chips at once as they won't fry properly.
Lower the chips into the fat and cook for 8 minutes, or until the chips are soft when pierced with a small knife. Lift out and set aside while you cook the rest. Increase the heat of the oil to 180°C, or until it's hot enough for a piece of bread dropped in to turn brown within 20 seconds.
Put some flour on a plate. Dust one piece of fish in flour, coating it lightly all over, then shake off any excess. Dip in the batter to coat thickly. Put in the fish in oil and fry in the deep fryer for 8-10 minutes, until golden brown and crisp. Keep warm in the oven on a lined baking tray. Repeat with the rest of the fish. (If you have room in the pan, you could cook two pieces at a time.)
When all the fish is cooked, fry the chips a second time in the deep fryer in batches, until crisp and brown. Drain on kitchen roll. Season with salt and British malt vinegar, then serve with the fried fish, lemon wedges, peas, and a tasty tartar sauce. There you have it: a real British-style fish dinner!