Put the dried chickpeas in a bowl, add cold water to cover by a generous 10 cm (4 inches) and leave to soak overnight.
Drain the chickpeas, transfer to a food processor and process until coarsely ground. Add the onion, garlic, herbs, ground coriander, baking powder and salt and pepper and process until really smooth. Using wet hands, shape the mixture into 16 small patties.
Heat a little vegetable oil in a large frying pan, add the patties, in batches, and fry over a medium-high heat for 3 minutes on each side until golden and cooked through. Remove and drain on kitchen paper.
Split the pitta breads and fill with the felafel, salad leaves and diced tomatoes. Add a spoonful of Greek-style yogurt and serve immediately.