A day ahead if possible, mix the spice mixture and rub over the lamb, leave in the fridge overnight.
Heat the oven to Gas Mark 5, 190°C (375°F).
Remove the lamb from the fridge 20-30 minutes before cooking to allow it to get to room temperature. Place the lamb in a roasting tin and season. Add a large cup of water to the roasting tin then place, uncovered, in the oven and roast for 1¼ hours. The lamb will be cooked medium at this stage. Remove from the oven, wrap loosely in foil and allow to rest for 15 minutes before carving. Boil up the juices in the roasting tin and stir in the redcurrant jelly. Serve with the sliced lamb.