1. Preheat the oven to 200°C, gas mark 6. Mix together the olive oil, ground ginger and paprika with seasoning and brush over the chicken skin. Put the lemon in the cavity of the chicken and place the bird in a roasting tin. Roast in the oven for 45 minutes per kg plus 20 minutes – about 1 ½ hours – until the chicken is cooked through with no pink meat and until the juices of the thigh run clear when pierced with a skewer. Rest for 10 minutes before carving.
2. While the chicken is cooking, make the mayonnaise. Beat the egg yolks with the garlic and mustard using an electric hand-held whisk. Gradually drizzle in the oil, beating as you go so that the mixture stays thick and creamy. Stir in the vinegar and seasoning then fold in the ginger. Pick the leaves from the watercress and chop, then stir into the mayonnaise. Serve with the chicken and steamed new potatoes and green beans if you like.