Preheat the oven to 160c/gas3. Toss the lamb in the seasoned flour then shake off any excess. Heat the oil in a large flameproof casserole or frying pan, add the lamb and fry quickly stirring frequently until meat is sealed.
Add the onion, celery, leek and carrots to the pan and fry quickly for a few minutes.
Layer the meat, vegetables and half the potatoes in a large casserole, adding the herbs and seasoning. Top with a layer of potatoes, overlapping each other and pour over the stock - just up to the top potato layer.
Cover with a tight fitting lid and cook for 1 1/2 hours.
Remove casserole from oven and turn temp up to 200c/gas6.
Melt the butter. Remove lid from casserole and brush potatoes with melted butter. Return to the oven for a further 20 minutes until golden and crispy. Sprinkle chopped parsley over top and serve.