Preheat the barbecue or grill to high. In a large bowl, mix together the olive oil, mint sauce and lemon juice. Add the lamb, pepper and courgette and mix to coat in the sauce.
Once coated, thread the lamb and vegetables onto 8 wooden skewers. Cook on the barbecue or under the grill for 10-12 minutes, turning occasionally and brushing with any of the mint mixture left in the bowl, until the lamb is cooked and the vegetables are beginning to char at the edges.
Warm the pitta breads on the barbecue or under the grill. To serve, open the pitta breads to make a large pocket, then add the grilled lamb and vegetables (removed from the skewers) and the salad leaves. Serve with the tzatziki.
Cook's tip: soak the skewers in water for 30 minutes to prevent them from burning on the grill.