CHICKEN ROASTED WITH GRAPES AND LEEKS

4 British chicken legs, trimmed of excess fat • 2-3 leeks, trimmed and sliced (about 350g prepared weight) • 150g essential Waitrose Green Grapes, halved • 4 rosemary sprigs • 100ml fresh chicken stock (or white wine or water) • Squeeze lemon juice • 700g essential Waitrose Mashed Potato • Handful rocket leaves, to serve