Preheat the oven to 180°C (350°F), Gas mark 4. Place the flour in a shallow dish and season generously, then use to coat the lamb. Arrange half of the lamb in the bottom of a round casserole dish and add a sprinkling of thyme. Scatter over the onion, followed by the carrots, then season to taste and add another sprinkling of thyme. Arrange the remaining lamb on top to cover the vegetables completely and sprinkle over the remaining thyme.
Pour enough chicken stock to just come up above the last layer of lamb. Add the Worcestershire sauce. Cover the casserole with a lid and place in the oven for about 1 hour until the lamb and vegetables are just tender and the stock has thickened slightly.
Place the potatoes in a large pan of boiling salted water and bring to the boil, then simmer for 10 minutes. Drain in a colander and set aside for 5 minutes until cool enough to handle. Slice the potatoes lengthways into 1cm (½in) thick slices and lay them in a slightly overlapping layer on top of the stew.
Melt the butter in a small pan or in the microwave and brush over the potatoes. Season to taste and cook in the oven for another 40 minutes until the potatoes are cooked through and nicely golden and the lamb stew is bubbling up around the edges of the dish. Serve straight to the table.