After marinating overnight, when you are ready to cook the lamb preheat the oven to 140C.
Roll up the shoulder of lamb and tie it tightly with string to keep it in a neat tube shape.
Place the celery, onion, bay leaves, and peppercorns in an oven-proof dish or meat tray.
Place the tied meat on top and pour over half of the marinade plus 75ml stock (chicken or lamb).
Bake the meat in the oven for 2½ hours.
Once cooked, remove the meat from the liquid and let it cool for half an hour.