Preheat the grill to medium. Thread the lamb cubes onto the skewers leaving a space between each piece and place under the grill. Cook for around 5-6 minutes, turning halfway through cooking, until tender. Alternatively, cook in an oven preheated to 200C/400F/Gas 6 for eight minutes. Baste with the melted butter and cook for another minute or two for slightly pink meat, or longer if you want the lamb well done.