Heat 1 tbsp of the oil in a large, deep frying pan and fry the leeks over a medium heat for 5 minutes until softened. Add the mince and fry, stirring all the time, for 6-7 minutes until no longer pink. Crumble over most of the Oxo lamb stock cubes (reserve about ¼ of one stock cube) then stir in the tomato purée and 300ml water. Bring the mixture to the boil then reduce the heat and simmer for 25-30 minutes, stirring occasionally. Add the frozen peas and sweet corn for the last 5 minutes of cooking time.
Meanwhile, cook the potatoes in a large pan of boiling water until just tender. Drain well and mash until smooth with the milk and butter. Season to taste. Preheat the oven to 200°C, fan 180°C, gas 6. Place four individual pie dishes on a large baking sheet.
Divide the mince mixture between the pie dishes and top with the mashed potato. Crumble the remaining Oxo cube over the potato topping. Drizzle with the rest of the oil. Bake in the preheated oven for 30-35 minutes until the filling is bubbling and the mash is golden. Serve immediately.