Soak the chickpeas overnight in cold water. Drain the chickpeas, rinse well and drain again. Put them into a large saucepan, cover with plenty of cold water and bring to the boil. Simmer for 1—1½ hours or according to the instructions on the packet until cooked and soft. Add extra water if necessary. Drain and allow to cool.
Put the tomatoes, celery, spring onions, olives and chickpeas into a large serving bowl and mix well. Stir in the dressing, season with black pepper, garnish with mint leaves and serve.