To make your mushy peas, put the butter in a pan with the peas and the chopped mint. Put a lid on top and simmer for about 10 minutes. Add a squeeze of lemon juice and season with salt and pepper. You can either mush the peas up in a food processor, or you can mash them by hand until they are stodgy, thick and perfect for dipping your fish into. Keep them warm while you cook your fish and chips.
Pour the sunflower oil into your deep fat fryer or a large frying pan and heat it to 190ºC/375ºF. Mix the salt and pepper together and season the fish fillets on both sides. This will help to remove any excess water, making the fish really meaty. Whisk the flour, beer and baking powder together until nice and shiny. The texture should be like semi-whipped double cream (i.e. it should stick to whatever you're coating). Dust each fish fillet in a little of the extra flour, then dip into the batter and allow any excess to drip off. Holding one end, lower the fish into the oil one by one, carefully so you don't get splashed – it will depend on the size of your fryer how many fish you can do at once. Cook for 4 minutes or so, until the batter is golden and crisp.
Meanwhile, parboil your chips in salted boiling water for about 4 or 5 minutes until softened but still retaining their shape, then drain them in a colander and leave to steam completely dry. When all the moisture has disappeared, fry them in the oil that the fish were cooked in at 180ºC/350ºF until golden and crisp. While the chips are frying, you can place the fish on a baking tray and put them in the oven for a few minutes at 180ºC/350ºF/gas 4 to finish cooking. This way they will stay crisp while you finish off the chips. When they are done, drain them on kitchen paper, season with salt, and serve with the fish and mushy peas.