Preheat the oven to 250℃/480℉ (fan-forced).
Prepare the baste by mixing all of the ingredients together in a small bowl.
Place the lemon halves in a large roasting tray, flesh side up. Sit the lamb on top of the lemons (so the lemon acts like a rack).
Pierce the flesh of the lamb with a pairing knife 12-14 times. Push the slivers of garlic into the holes.
Rub the lamb with the marinade and add the water to the bottom of the tray.
Now cover with foil making sure the edges are sealed well. I use at least 3 pieces of foil and tuck it under the tray so it is firmly sealed. The lamb needs to steam to make it fall apart when it's done. Any gaps in the foil it can dry out so take your time and seal it carefully.
Put it in the oven and immediately turn it down to 150℃/ 290℉. Cook for 4 hours.
Whilst that's cooking make the chutney by combining the mint, garlic, chilli and salt in a food processor and processing to a paste. Mix in the lemon juice and vinegar and store in the an airtight container in the fridge until the lamb is ready to serve.
When the lamb is ready, remove the foil and put it under the grill for 5-10 minutes or until the top is crispy.
Rest the lamb for a few minutes before serving with the mint chutney and tongs to pull apart the meat.