Preheat the oven to 180°C/gas mark 4. In a bowl, mix the butter with the herbs, garlic and cumin. Rub over the shanks.
Cut out 2 foil squares and divide the carrots and leeks between each. Fold up the foil to make a bowl. Place the shanks on top of the veg, then carefully pour in the wine. Gather the foil around the bone, pinching it together tightly. Put on a baking tray and roast for 2 hours, until the lamb falls off the bone.
Meanwhile, put the potatoes in a pan of cold water. Bring to the boil and simmer for 10-12 minutes until tender. Drain, then mash with the butter and milk. Serve the lamb with the mash and vegetables.