Preheat the oven to 200ºC/Gas 6.
Place joint, fat side up in a roasting pan on a rack and rub with salt and pepper.  Make small incisions in the joint with the point of a knife, rub the mustard into the meat and insert the garlic into the incisions.  Pour enough cold water into the pan to cover the base.

Add the thyme to the water and place in the centre of the oven. Cook for 10 -15 minutes then cover  with tin foil and reduce the temperature to 180ºC/Gas 4, cooking for a further 20 minutes per 450g.  Add more water as necessary to keep liquid in the bottom of the pan and baste occasionally.

After cooking allow the meat to rest for 15-20 minutes before carving.  Use the remaining pan juices to make a delicious gravy or sauce.