First clean and gut your swan, then cover the outside of the bird with olive oil. Roast it on a spit or, failing that, in the oven. Baste frequently with its own juice and when it is done carve and serve in pieces.
However, before heading off down to the local butcher’s to ask for swan, you should bear in mind that all swans are protected under the Wildlife and Countryside Act of 1981. The mute swan is even better protected because the species is owned by the Crown, and has been ever since 1482. A small number of shooting licences are granted to farmers each year, if they can prove that swans have damaged their crops, but in all other cases it is an offence to be in possession of a swan carcass, even if the bird died of natural causes!