Melt the butter in a frying pan over a low heat. Stir in the cornflour until you have a smooth paste and cook, stirring all the time, for a minute. Slowly whisk in the strained liquid, a little at a time, until smooth. Simmer for 10 minutes over a low heat, stirring frequently, until thickened. Add any juices that have come out of the lamb while resting and stir well. Carve the lamb and serve with the gravy and your favourite sides.