First heat oven to 200C/fan 180C/gas 6. Then boil potatoes for 10-15 mins until tender.
Put the tomatoes onto a baking tray, rub with a little oil and some seasoning, then roast for 5 mins until the skins are starting to split. Add the fish to the tray, top with most of the lemon zest and some more seasoning, then drizzle with a little more oil. Roast for 8-10 mins until the fish flakes easily.
Drain the potatoes well, then return to the saucepan over a low heat and mash – leaving the heat on will help to dry out the potatoes for a fluffier mash. Stir in the lactose free milk, then add the basil.
Serve the fish on top of the basil with the vine tomatoes draped over the top.