Heat the oil in a medium saucepan, then soften the onion and carrots for a few mins. When soft, turn up the heat, crumble in the lamb and brown, tipping off any excess fat. Add the tomato puree and Worcestershire sauce, then fry for a few mins. Pour over the stock, bring to a simmer, then cover and cook for 40 mins, uncovering halfway.
Meanwhile, heat the oven to 180C/ fan 160C/ gas 4, then make the mash. Boil the potatoes in salted water for 10-15 mins until tender. Drain, then mash with the butter and milk.
Put the mince into an ovenproof dish, top with the mash and ruffle with a fork. The pie can now be chilled and frozen for up to a month. Bake for 20-25 mins until the top is starting to colour and the mince is bubbling through at the edges. (To bake from frozen, cook at 160C/fan 140C/gas 3 for 1 hr-1 hr 20 mins until piping hot in the centre. Flash under the grill to brown, if you like.) Leave to stand for 5 mins before serving.