Place the couscous, celery, tomatoes and salt in a bowl, add the water. Cover and leave to stand for approx. 5 mins. Separate the couscous with a fork.
Combine the balsamic, oil, parsley, salt and pepper. Mix the dressing and diced apple in with the couscous.
Scrub the mussels thoroughly under cold running water. Use scissors to cut off the beard where necessary. Discard any mussels that are already open as these are inedible. Heat the oil in a wide pan. Add the garlic, sauté for approx. 1 min., add the mussels, cover and cook for approx. 1 min. Pour in the gin, cover and simmer for approx. 4 mins. until the mussels open, stirring once. Discard any mussels that have not opened as these are inedible. Pour the mussel water through coffee filter paper, retain the liquid, add to the salad along with the mussels.