Put the flour in a large bowl and season well. Toss the lamb in the seasoned flour so it's well coated. Heat 2 tablespoons of the oil with the butter in a large flameproof casserole dish or deep ovenproof sauté pan.
Tip in the lamb and fry over a high heat for 5 mins, turning over occasionally, until no longer pink and starting to brown. Transfer to a plate with a slotted spoon.
Add the diced vegetables except the potato to the dish or pan with another tablespoon of oil if needed, and fry over a medium-high heat for 5 mins, or until just starting to brown, stirring occasionally. Stir in the garlic and cook for 1 min, then the tomato purée and keep stirring for about 30 seconds. Pour in a little of the red wine and scrape the bottom of the pan to deglaze it. Slowly add the rest of the wine and the stock, stirring all the time.
Return the lamb to the pan along with the potato, mushrooms, rosemary and thyme. Cover and cook in the oven for 1¾ hours.
To Make the Dumplings
Put the flour in a medium bowl and season with salt and pepper.
Add the suet and rub together with your fingers to make rough crumbs. Stir in the rosemary, thyme and parsley, then pour in 2 tablespoons of cold water and mix using a round bladed knife to a soft dough. Cut the dough into 8 even-sized pieces and shape into balls.
Gently place the dumplings on the surface of the casserole and cook in the oven, uncovered, for 20 more mins until theyare risen and the lamb is meltingly tender.