Season the lamb joint all over. Set on a roasting rack and roast fro 1hour 40mins at 180C/170C fan for medium cooked meat. Rest for 15 minutes.
20 minutes before the end of the cooking time mix togerther mint jelly and the lime zest and juice, seaon and spoon over the lamb, reserving some to serve separately.
Skim off any excess fat from the roasting tin and discard. Over a medium heat, stir in the flour and deglaze with some of the lamb stock. Add remaining lamb stock until desired consistency is reached.
Simmer for 2-3 minutes until thickened. Check the seasoning. Stir the chopped mint into the hot sauce.
Serve the lamb and the sauce on hot plates with roasted potatoes, steamed broccoli and offer the mint and lime jelly separately.