Tip the peppercorns into a small heavy-based saucepan. Heat gently for 1 minute, and then add the salt. Toast for a further 1-2 minutes. Place in a pestle and mortar and crush to a fine powder or allow to cool slightly, then place in a plastic bag and crush with a rolling pin.
Put the salt and pepper into a mini blender with the garlic, rosemary, mint, olive oil, lemon juice and zest then blitz to form a smooth paste. Place the butterflied leg of lamb into a sealable plastic bag, pour over the paste and rub all over, by massaging the lamb inside the
bag. Leave to marinate for at least 2 hours but preferably overnight covered with cling film in the fridge. Ensure you give the meat a good "massage" every few hours.
Barbecue the lamb over medium-hot coals for about 40 minutes for medium-rare lamb, turning occasionally. Lift the cooked lamb on to a board, cover with a sheet of foil and leave to rest for 5-10 minutes, then carve into thin slices and serve with the cous cous salad. Alternatively roast at about 180C for a similar amount of time.