In a saucepan, combine cider vinegar, mint jelly, brown sugar, sugar, butter, and lemon juice.

Bring the mixture to a boil, stirring until sugars are dissolved.

Add dry mustard and salt to the sauce.

Pour the marinade over the butterflied leg of lamb, ensuring it's well coated.

Cover the lamb and marinate in the refrigerator for at least 3 hours, or preferably overnight.

Before grilling, remove the lamb from the marinade and reserve the marinade.

Grill the lamb over medium heat, turning occasionally, until cooked to desired doneness.

Baste the lamb with the reserved marinade during the last few minutes of grilling.

Let the lamb rest for 10 minutes before carving and serving.