1.Combine marinade ingredients in a shallow roasting pan and season. Add lamb, turning to coat well. Turn skin-side up, cover and chill for 1 hour.
2.Preheat barbecue to medium-high and bring the lamb back to room temperature. Half-fill a deep-fryer or large saucepan with oil and heat until 120°C on a kitchen thermometer.
3.Rinse potato under cold water to remove starch, then pat dry with paper towel. In 2 batches, deep-fry for 10 minutes or until just tender but not coloured. Remove with a slotted spoon and drain on paper towel. Increase oil heat until 190°C on a kitchen thermometer. In 2 batches, deep-fry chips for a further 6-8 minutes until crisp and golden. Remove with a slotted spoon, drain on paper towel and sprinkle with sea salt. If necessary, keep chips warm in an oven preheated to 150°C. Meanwhile, barbecue lamb, for 10-12
4.Meanwhile, barbecue lamb, for 10-12 minutes each side for medium-rare (a leg of lamb contains a good deal of fat, so you’ll need to keep moving it to different parts of the barbecue to avoid excessive flare-ups, which would cause the meat to burn). Transfer lamb to a board, cover loosely with foil and rest for 10 minutes.
5.Carve the lamb into thick slices and serve with olive oil chips and tomato salad.