Preheat the oven to 190C/170C Fan/Gas 5.
Prick the potatoes all over with a fork, then rub with a little oil and bake for 1-1½ hours, or until tender.
Heat a large pan until hot, add the vegetable oil and fry the beef in batches until just browned. Add the onions, garlic, thyme and celery and fry for a couple of minutes until softened.
Add the tomato purée and cook for 1-2 minutes then add the Worcestershire sauce and the finely chopped carrot. Add the red wine and cook until reduced in volume by a third, then add the beef stock and bring to a simmer, stirring occasionally. Simmer for 30 minutes until the beef is tender and the sauce has thickened.
Meanwhile, put the remaining carrots in a pan of water, add a pinch of salt, the sugar and a knob of butter. Bring to the boil, then reduce to a gentle simmer and cook for 30-40 minutes until soft.
Once cooked, tip the mince into a bowl. Season to taste with salt and black pepper and as much Worcestershire sauce as you like.
Set aside to cool while you make the mash.
When the potatoes are cooked, take them out of the oven to cool. Turn the oven up to 220C/200C Fan/Gas 7.
When the potatoes are just cool enough to handle, slice them in half, scoop out the flesh and pass through a potato ricer into a bowl, or mash them with a fork.
Add half the remaining butter to the potatoes and warm the milk in a saucepan until just simmering. Pour onto the potatoes and beat until the mash is smooth and creamy. Season to taste with salt and black pepper.
Pour the cooled mince into a baking dish and spoon the hot mash over the top, fluffing the top with a fork. Scatter a few dots of butter over the top then place in the oven for 10-15 minutes until golden brown and piping hot through.
Serve with the cooked carrots.