Prick the duck legs all over on the skin side and then rub each leg, on both sides with 1 tsp five spice powder. Season, on the skin side with salt and pepper.
Place on a rack in a roasting dish skin side up, cover with foil and roast in the oven for 1 hour, 50 minutes.
Meanwhile, put the plums, sugar, star anise and cinnamon into a small pan and place over a moderate heat with the lid on. Leave to cook gently for about 30 minutes. The plums will break down and form a soft, sticky chutney, set aside to cool slightly. Once cooler, remove the star anise and cinnamon before serving.
Remove the duck from the oven and allow to rest for 10-15 minutes. Whilst the duck is resting, cut the bun in half and toast either on a griddle or in a toaster.
Using two forks, pull the duck meat from the bones and pull into small pieces like for a Peking duck. Put an equal amount of the salad leaves onto the bottom half of the four buns, Pile the duck onto the leaves and top with a large dollop of the plum sauce, Put the top of the bun on and serve.