Heat some dripping or 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 dripping 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 dripping. Cover, then place in the slow cook for about 3 hrs on high until the potatoes are cooked.
Remove the lid, brush the potatoes with a little more dripping.
You can finish under the grill for 5-8 mins until the potatoes are brown.