Heat the oil in a pan and fry the onions and garlic softly for 10 minutes. Add the mince to the pan and cook on a higher heat, stirring contantly with a wooden spoon, until the mince is broken up and well browned.
Stir in the tomato puree and flour and cook for 1 min. Add the Worcestershire sauce, stock and bay leaves. Cover, bring to the boil and season generously. Reduce the heat and simmer for 30-40 mins until the meat is tender. Remove the bay leaves and discard, then spoon the mixture into a 1.8 litre ovenproof dish.
Meanwhile, cook the potatoes in a large pan of boiling, salted water. Drain well, tip the potatoes back into the pan and heat for 1 min to dry off. Add the skimmed milk, season with salt and freshly ground black pepper and mash well. Preheat the oven to 200 C / gas mark 6.
Spoon the mash on top of the mince, spread over evenly and rough up the surface with a fork. Put the dish on a baking sheet to catch any spillages and cook for 20-25 mins until golden and bubbling.
This pie freezes well - simply assemble the pie and freeze before baking. Once you're ready to bake, simply throw it in the oven straight from the freezer.