Using a small knife, cut away any excess fat covering the leg. Cut through the tendon joining the meat to the knuckle and trim off any meat from the bone. Using a saw, cut off the knuckle end of the shank bone to neaten it.
Next, turn the leg over and, using a boning knife, cut around the edges of the pelvic bone. Following its contour down to the ball and socket joint, cut through the tendons that join the two bones and then pull out the pelvic bone.
Fold the flap of flesh back over the lamb and use a piece of string to tie it in place. Wind the string round the shank end and up both sides of the leg. Tie another piece of string across the joint.