Start with mixing the marinade. Pour the balsamic vinegar into a large mixing bowl. Grate the garlic directly into the vinegar then add the wholegrain mustard and combine with a wooden spoon until thick. Add the jam followed by the chopped herbs. The mixture should be thick and dark. Finally add the salt, pepper and oil, and then combine.
Prepare the meat. Open out the leg of lamb on a chopping board. Make some shallow score marks through the fat of the lamb taking care not to cut too deeply in to the meat. Trim back any large excess pieces of fat then add the meat to the bowl. Coat the leg meat in the thick marinade until evenly distributed. Marinade the meat for at least 1 hour. Remember the longer you leave it the stronger the taste.
Roast (in-direct method) for 1 hour 10 mins using the Premium Grill Rack. The best results are achieved by cooking at a lower temperature ideally 165°C. If you like it well done you may want to extend the cooking time by 20 minutes.
Allow the meat to rest for 15 minutes before you serve.