1. Place the tuna steaks in a shallow dish.
2. Mix together the soy sauce, Worcestershire sauce, mustard, sugar and oil in a small bowl. Pour the marinade over the tuna steaks.
3. Gently turn over the tuna steaks, using your fingers or a fork, so that they are well coated with the marinade.
4. Cover and place the tuna steaks in the refrigerator and leave to chill for up to 2 hours.
5. Barbecue (grill) the marinated fish over hot coals for 10-15 minutes, turning once. Baste frequently the marinade.
6. Garnish with flat-leaf parsley and lemon wedges, and serve with a fresh green salad.