Mix together the yoghurt, oil, lemon, garlic, herbs, salt and pepper.
Rub the marinade all over the lamb and place in the fridge for at least 2 hours or longer, up to overnight.
Remove the lamb from the fridge 30 minutes before cooking and preheat barbecue to hot.
Scrape any excess yoghurt marinade off the lamb and grill for 5 minutes on each side over a hight heat. Reduce the heat to low and continue to cook the lamb for 15 minutes for medium-rare. Increase this time to 25 minutes for well-done and reduce to 10 minutes for rare.
Rest the meat, covered, for 20 minutes.
Make the tzatziki by simply mixing all of the ingredients together.
Slice the lamb and drizzle it with olive oil.
Serve with a big dollop of tzatziki, wedges of lemon, grilled slices of haloumi and plenty of fresh salad.