One hour before you want to cook it, take the lamb out of the fridge. Remove the packaging and sit it in a snug roasting tin. Heat the oven to 140°C/Fan 120°C/Gas 1.
Peel and crush or grate the garlic. Scrape into a small bowl. Finely chop the leaves from the rosemary and sage sprigs and add them to the garlic. Pour in the vinegar and add a good pinch of salt and pepper. Stir together.
Use a small, sharp knife to cut shallow slashes all over the lamb. Rub the herby mix into the lamb, make sure the herbs and garlic are worked in and push some into the ends of the lamb too. Cover the tin with foil, crimping the foil to the edges to the tin to seal it.
Roast the lamb in the oven for 3 hrs. Then take the foil off, turn the heat up to 180°C/Fan 160°C/Gas 4 and roast the lamb for 30 more mins to brown it.
Take the lamb out of the tin, sit it on a board and loosely cover with foil. Rest for at least 30 mins before carving.