1. To make the marinade: mix 50ml dark soy sauce with the olive oil. Add the rosemary, thyme and garlic and pour over the duck breasts. Leave to marinate for 20 minutes.
2. Preheat the grill.
3. Remove the duck breasts from the marinade and grill for 5-6 minutes until pink.
4. Place the duck breasts in a baking dish.
5. Preheat the oven to 180°C/gas 4.
6. To make the plum sauce: mix together the plums, remaining soy sauce and honey and pour over the breasts. Cook for 30 - 40 minutes until the duck is cooked. The plum stones should have dropped to the bottom of the dish.
7. To serve, slice the duck breasts and arrange on plates. Spoon over the sauce.