Cook the potatoes in a large saucepan of boiling water for about 20 minutes until tender.While the potatoes are cooking, heat a large saucepan. Add the minced beef a handful at a time, cooking until browned.Stir in the onion, carrot, courgette, green beans and mixed herbs, then add 450ml water. Bring to the boil, then turn down the heat and simmer without a lid for about 20 minutes, until the veg has softened.Turn the grill on to preheat, and warm a large baking dish under it for a couple of minutes. While the grill is heating, drain and mash the potatoes, seasoning them with some pepper.Add the sweetcorn to the mince mixture, then sprinkle in the gravy granules and stir until thickened. Season if needed, then tip into the baking dish. Spoon the mash on top, spreading it out to cover the mince, then pop under the grill.Once the top has browned, remove from the grill and leave to cool for a couple of minutes. Dish up and enjoy!