Using a sharp knife cut through the narrow end of the lamb leg about 3 inches from the end. Cut through the meat around the bone. Remove and discard this piece, leaving clean exposed bone. You may be able to have your butcher do this for you. Stick cloves into the lamb leg in an even pattern then place into a shallow metal roasting pan.
Bake in preheated oven for 2 hours then drain off fat and drippings. Whisk together apricot nectar, salt, pepper and soy sauce in a small bowl; pour over lamb leg. Return the lamb to the oven and bake until medium-well doneness, basting frequently. Top the roast with lemon slices and bake 5 minutes longer.
Remove the lamb to a serving platter and cover with aluminium foil. Allow to rest 10 to 15 minutes before slicing. Meanwhile place the roasting pan on the stove over medium heat. Dissolve the cornflour in the water and pour into the roasting pan along with the stock cube. Cook and stir until the cube dissolves and the sauce thickens and clears, about 1 minute. Remove the cloves and lemon slices before slicing the lamb and serving with the sauce.