Pound the conch meat with a mallet until tender. Chop into bite-size pieces. Place in a bowl, sprinkle with lemon juice, and set aside.
Heat the oil in a frying pan over medium heat, and cook the garlic, onion, green pepper, red pepper, celery and mushrooms until tender. Mix in the tomatoes, Worcestershire sauce, ketchup and sugar. Season with red chilli flakes, salt and pepper. Bring to the boil. Reduce heat to low, and simmer uncovered 20 minutes, stirring occasionally.
Mix the conch meat into the frying pan, and continue to cook 10 minutes. Do not overcook, or the conch will become tough.
Conch can be found in some Caribbean markets.
The sauce may be prepared ahead and refrigerated 3-4 days or frozen until ready to use. Simply reheat, add conch, and serve.