Heat oven to 160C. Heat some butter in a large shallow casserole dish, brown the lamb in batches, lift to a plate, then repeat with the kidneys.
Fry the onions and carrots in the pan with a little more butter until golden. Sprinkle over the flour, allow to cook for a couple of mins, shake over the Worcestershire sauce, pour in the stock, then bring to the boil. Stir in the meat and bay leaves, then turn off the heat. Arrange the sliced potatoes on top of the meat, then drizzle with a little more butter. Cover, then place in the oven for about 1½ hrs until the potatoes are cooked.
Remove the lid, brush the potatoes with a little more butter, then turn the oven up to brown the potatoes, or finish under the grill for 5-8 mins until brown.