Preheat the oven to 220oC/425oF/gas mark 7. Weigh the lamb and calculate the cooking time at 20 mins per 450g plus 10-20 mins, depending on how pink you like your meat. Peel and slice the garlic into thin slivers. With the point of a knife, make small slits all over the meat and push a piece of garlic into each cut. Season.
Cook in the oven for 15 mins and then lower the temperature to 180oC/350oF/gas mark 4 and continue cooking for the rest of the cooking time.
Meanwhile, finely chop the cucumber and remove excess water by placing on a paper towel. Mix together the yoghurt and Mint Jelly and stir in the cucumber. Shred the lettuce.
When the lamb is cooked, allow to stand for 10 mins before carving. During this time, warm the pitta bread in the oven.
Carve the lamb into generous slices and serve in the warm pittas with the sauce and the lettuce.