Mix the mince with half the chopped basil, the Worcestershire sauce and some seasoning, then use your hands to shape into 16 meatballs.
Heat a non-stick frying pan, add the oil, then fry the meatballs, turning, for 5 mins until browned. Tip in the passata and remaining chopped basil. Simmer gently for 10 mins until the meatballs are cooked through.
Meanwhile, boil potatoes for 10-15 mins until tender. Drain 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 marble through the pesto. Pile the mash onto 4 plates, spoon over meatballs and serve scattered with the reserved basil.