Remove the lamb from the refrigerator about 2 hours before you intend to roast it. Using a fork, beat the softened butter with the rosemary and thyme. Gradually beat in the lemon juice, then continue to mix until smooth. Season generously with freshly ground black pepper. Spread the seasoned butter over the entire surface of the joint. Leave it to stand at room temperature until it has lost its chill. Heat the oven to 170C / 325F/gas3.

Place the lamb, fat side up, on a rack set over a roasting tin. Roast in the oven for 1 1/2 hours for pink meat, or about 1 3/4 hours for well done. Baste the meat occasionally.

Transfer the roast lamb to a well-heated serving platter and leave it to stand in a warm place for 10-15 minutes to settle before carving.

Meanwhile, make the gravy. Skim the excess fat from the drippings in the roasting tin. Place the tin over direct heat, add the beef stock and tomato puree and bring the mixture to the boil, stirring and scraping the bottom and sides of the tin with a wooden spoon to dislodge the crusty bits.

Make a beurre manie with the butter and flour by mashing them together to a smooth paste. Add to the boiling liquid in tiny pieces, stirring until they have dissolved into the gravy. Simmer for a further 2—3 minutes, until the gravy has thickened. Season, strain the gravy into a heated sauce-boat and serve with the lamb.