Set the oven to Gas Mark 6, 200C (400F)
Spread the fat surface with a mixture of mustard and freshly ground black pepper. Place the meat in the roasting tin. Add some peeled shallots or onion halves around the joint to give some extra flavour and colour to the gravy.
Place the joint in the hot oven for 15 minutes, reduce the temperature to Gas Mark 4, 180C (350F) and allow 15 minutes per half kg for rare, plus 20 minutes extra for medium or 30 minutes extra for well done. To be absolutley accurate use a meat thermometer. When the meat is cooked remove from the oven, cover loosely with foil and allow to rest for 30 minutes. This resting time allows the juices to settle; the meat firms up so it is easier to carve.
Spoon off the fat. Add some wine or water to the juices. Boil over a high heat and taste for seasoning. Serve with the sliced meat. horseradish cream, crispy roast potatoes and either roast vegetables or a creamy vegetable puree.