Turn the slow cooker on to your preferred setting.
Heat a large frying pan over a high heat. Rub 1 tbsp of oil all over the lamb and season generously with sea salt and freshly ground black pepper. Sear the lamb in the pan until evenly browned, then put in the slow cooker.
Deglaze the pan with a splash of madeira, scraping off any caramelisation on the base of the pan, and pour into the slow cooker.
Heat the remaining 1 tbsp of oil in the pan and brown the onions, garlic and celery until evenly golden. Add the remaining madeira and stock to the pan, bring to the boil, then tip into the slow cooker with the thyme, bay and prunes. Cover and cook for 5-6 hrs on high, or 7-8 hrs on low, turning the lamb halfway through, until the meat easily falls off the bone when pulled with a fork. Transfer to a serving dish and pull into large chunks. Discard the bone.
Drain the cooking juices into a jug. Skim off and discard any fat that rises to the top. Pour 500ml of the cooking liquid back into the slow cooker. Mix 3 tbsp of the liquid with the cornflour, then whisk back into the slow cooker and cook for 5 mins, stirring constantly, until thickened. Alternatively, cook in a pan over a medium heat for 2-3 mins until thickened. Season to taste with salt and freshly ground black pepper.
Drizzle a little of the gravy over a lamb, top with the prunes and garnish with pomegranate seeds and parsley. Serve with a jug of the remaining gravy.