Combine the marinade ingredients in a large non-metallic dish. Add the diced lamb and stir to coat well, then cover and place in the fridge for about 2 hours if possible. Meanwhile soak 8 wooden skewers in water. This will prevent them scorching. If you are short of time use metal skewers.
Preheat your barbecue or grill to high and brush lightly with oil. Thread the lamb cubes, peppers and red onion onto the skewers, season and cook for 3-4 minutes on each side until lightly charred and cooked to your taste.
To make the Mint and Garlic Yoghurt Dressing, stir together all the ingredients and season to taste. Drizzle a little dressing over each lamb skewer and serve the rest on the side.