Grate the courgettes in a food processor and combine in a bowl with the egg, flour and feta. Season. Meanwhile, heat the oil in a large nonstick frying pan over a high heat.
Drop several spoonfuls of the mixture into the frying pan and cook for 2 minutes on each side until golden (you may need to do this in batches).
While the fritters are cooking, make the herby yogurt. Finely grate the garlic then stir it into the yogurt along with the herbs. Season to taste. To serve, stuff the pitta breads with the fritters, tomatoes and lettuce and a spoonful of the herby yogurt.