Heat oven to 220C/200C fan/gas 7. Boil the potatoes until knife-tender, then drain. Spread out on a large baking tray and gently crush with the back of a spatula. Add the peppers, drizzle with the vinegar and oil, season well and roast for 25 mins.
Put the piri-piri ingredients in a food processor with some salt. Purée until fine, then pour into a bowl. Put the fish on a baking tray and spoon over some of the piri-piri sauce. Season and bake for the final 10 mins of the potatoes’ cooking time. Serve everything with the extra sauce and a green salad on the side.