Pulse canned tomato in a food processor until coarsely chopped. Transfer to a large saucepan over medium-high heat, add sugar, vinegar, Worcestershire sauce, mustard, garlic and spices and bring to the simmer, stirring occasionally, until thickened (7-8 minutes). Stir in butter and season to taste.
Heat a char-grill pan (or barbecue) over high heat. Brush prawns with a little olive oil and cook in batches, flesh-side down, for 1-2 minutes, then turn and brush generously with tomato mixture. Char-grill until prawns are just cooked though (2-3 minutes). Transfer to a serving plate, scatter with chives and serve hot with remaining sauce.