Heat the olive oil in a frying pan and brown the lamb for 4–5 minutes and add to the slow cooker. This adds flavour but isn't essential.
Put the potatoes, carrots, onion and garlic into the slow cooker
Mix the stock, tomato purée, Worcestershire sauce and thyme together. Pour into the slow cooker and add the bay leaf.
Cook on LOW for 7–8 hours or HIGH for 4–5 hours, until the lamb is tender and the potatoes are soft.
If the gravy is too thin, mix 1 tbsp cornflour with 2 tbsp cold water. Stir into the stew and cook on HIGH for another 20–30 minutes.