Place the lamb mince in a large pan, put the heat on high, break it up and stirring occasionally cook for ten minutes until it browns. Add the rosemary, onion, carrots, celery and garlic and cook for another five minutes until the vegetables begin to soften. Stir in the tomatoes with the tomato puree and Worcester sauce. Rinse the tomato can with around with two tbsp of water and add to the pan. Bring up to the boil and simmer for 15 minutes.
Meanwhile, place the whole sliced Jersey Royal potatoes in a large pan of lightly salted boiling water, bring back up to the boil and cook for ten to 15 minutes until just tender. Drain. Roughly crush using the back of a spoon, stir in the butter, mint and some freshly ground pepper and salt.
Preheat the oven to fan 160ºC. Divide the lamb mixture between four ovenproof dishes or make one large pie if preferred, top with the crushed potatoes and bake for 20-30 minutes until golden or until they turn crispy. Serve with steamed veg alongside.