Pre-heat the oven to 220°c. Put the carrot, celery and onion in a roasting tin and sit the mutton on top. Season with salt and pepper. Pour the beer and the stock around the mutton and put it in the oven. After half an hour, reduce the heat to 140°c and leave to cook for 4 hours, until the meat is falling away from the bone. Check the meat periodically and top up the liquid with more stock if needed. Remove the meat and vegetables to a warm plate and skim off any fat from the liquid in the roasting tin, then put it on the hob and heat until the liquid is bubbling. Stir in enough of the cornflour paste to thicken the gravy. Serve with creamy mashed potatoes and seasonal vegetables.