For the fish and chips, heat the oil in a deep-fat fryer to 190C, or half fill a deep heavy pan with oil and heat until a breadcrumb sizzles and turns brown when dropped in. (Caution: Hot oil can be dangerous. Do not leave unattended.)
Sprinkle the yeast and sugar into a mixing bowl. Pour over the beer and vinegar and whisk in the flour and salt. Leave to ferment. It is ready to use when the mixture starts to bubble.
Dip the fish in the batter and deep-fry until golden-brown and cooked through (about 3-4 minutes). Remove with a slotted spoon and drain on kitchen paper.
Add the potatoes to the deep-fat fryer and fry for 3-4 minutes, or until golden-brown and crisp. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper.
For the peas, place the peas, yoghurt, chilli flakes, salt and pepper into a blender and blend until almost smooth.
Mix in the mint just before serving and serve immediately with the fish and chips.