Make the pastry and chill for at least 1 hour before using. See that the lamb is fairly free of fat, and securely tied: put in a roasting pan and rub with about a third of the butter. Put into a very hot oven, 450F for 20-30 minutes. Take out and allow to cool slightly.

~- Miz the remaining butter with the herbs, salt and pepper. Roll out the pastry to a piece large enough to wrap around the joint. Carefully remove the string from the lamb, trying to retain the shape. Put it in the center of the pastry, dampen the edges, and draw the pastry up over the top: secure well by squeezing the pastry ends and edges together. Turn over so that the fold is underneath, and put on a baking sheet. Prick all over the top lightly with a fork, then brush with either the milk or beaten egg. Put into 400F oven and cook for about 1/2 hour, or until the pastry is nicely browned. Turn the sheet once during baking to make sure the browning is even. Serve, cut into fairly thick slices.