Place ribs in a shallow ceramic or glass dish so they lie flat.
Drain the plums and reserve the syrup.
Force plums through a sieve or puree them in a blender.
Add the reserved syrup, orange juice concentrate, plum sauce, Worcestershire sauce and garlic.
Mix well, then pour over ribs.
Marinate ribs for at least 1 hour.
Drain ribs and reserve marinade.
Place ribs on a well-oiled grill over medium-hot coals.
Brush them with half the plum sauce and cook for 10 minutes.
Turn ribs carefully, then brush with the remaining sauce.
Cook for a further 20 minutes, or until tender.