Make the marinade: mix the sugar, lemon juice and zest with the olive oil, balsamic and some seasoning. Stir to dissolve the sugar then mix in the garlic and mint. Reserve half of the marinade to use as a dressing for the lamb later.
Open out the meat and lay it in a deep tray. Pour the marinade on top then turn to coat all over in the mix. Cover with cling film and leave for 1 hr or overnight in the fridge.
Remove the lamb from the fridge and shake off any excess marinade back into the tray. Insert a skewer so that it goes from one corner to the opposite one. Do the same with another skewer in the opposite direction – this makes it really easy to turn the meat on the barbecue.
Heat a barbecue or griddle pan until it’s a medium heat. Cook the lamb for 15-20 mins, turning frequently for pink lamb or longer if you prefer it more done. When cooked, rest for 5-10 mins before taking out the skewers, slicing and serving with the reserved marinade for pouring over.