Trim all the fat off the lamb and pull off all the skin. Make deep incisions all over the lamb with a sharp knife.Mash the butter with the garlic and the spices ( not salt) and rub it all over the lamb, pushing it well into the holes in the meat.
Loosely cover with cling film, put it in the refrigerator and leave for about eight hours or better still overnight. Return to room temperature before roasting.
Season the meat well with salt. Roast in an oven preheated to 220 C, 200 Fan, Gas 7 for 15 minutes. Turn heat down to 180 C, 160 Fan gas 4 and cook for another 1 1/2 hours.
Remove the lamb to a warm platter, cover with foil and a couple of tea towels to insulate and leave to rest for 15 minutes. Serve on a big warm platter.