Place lamb in slow cooker.
Pour an inch of hot water from the kettle into the bottom of the slow cooker and add the garlic and rosemary.
Switch on the slow cooker and let cook for 4-6 hours on high or 8-10 hours on low.
When it is easily shredded with a fork, remove onto a plate and pour the juices into a small saucepan. Remove the rosemary sprigs and discard.
Smash the garlic cloves in the juice to break them up, and add the mint sauce salt and pepper.
Bring the juices to a boil. Cook until the sauce has reduced by half.
Meanwhile, shred all the meat from the lamb joint. Pop into a bowl, and when the gravy is ready pour this over and mix well.