In a bowl, stir together the yoghurt spices, garlic, ginger, yoghurt and lemon juice.
Use a sharp knife to cut slits into the lamb and rub half the yoghurt mix all over the meat, making sure to massage it into the slits.
Carefully lift the lamb into a Magic Chicken Roasting Bag, place the bag on a tray secure the bag with a nylon tie and refrigerate overnight. Keep the remaining yoghurt mix and leave in the fridge until ready to use.
Preheat the oven to 180°C, 160°C fan, gas mark 4.
In a bowl, combine the remaining yoghurt mix with the potatoes. Spoon the potatoes into a Magic Chicken Roasting Bag, place the bag on a tray secure the bag with a nylon tie.
Before cooking, trim any excess bag. Use a scissors to make slits in the top of the bag.
Cook the lamb for 1½-2 hours. For the remaining 25-30 minutes, add the potatoes. Remove the oven and leave the lamb to rest in the bag for 30 minutes.
Cut the bag open and carefully lift the lamb onto a serving plate/platter. Remove the potatoes and serve alongside the lamb.
Sprinkle with some fresh coriander and serve with yoghurt and some warmed naan breads.