Preheat the oven to 200C/400F/Gas 6.
Place the duck legs into a roasting tin and season well with salt and freshly ground black pepper. Rub the five-spice powder into the skin of the duck legs, then place a bay leaf or two on top of the meat.
Cook in the oven for about one hour, until the skin is very crisp and the meat falls away from the bone.
Meanwhile, prepare the salad by peeling the grapefruits and oranges by cutting the ends off, placing them on a chopping board cut-side down, then running a sharp knife down the side of the fruits to remove the skin and white pith. Then run a knife down in between the membranes to segment the fruits, catching any juice in a bowl.
When the duck is cooked, strip the meat away from the bone and cut into bite-sized chunks. Toss the watercress with the segmented fruits and the fruit juice and serve topped with the duck meat.
To make a little dressing, drain off the fat from the roasting tin (you can keep it in the fridge and use it to make roast potatoes). Swirl a little of the juice from the oranges and grapefruit into the pan to lift the crusty bits from the bottom, and pour over the salad.