Roast beef isn't difficult to cook--the most important thing to do is make sure you've got the right cut of meat for roasting. The slow-roasting method is better for tougher cuts, like sirloin or the wing end of the sirloin (known as rib of beef) because the low heat of a roast prevents gristle from getting too tough. With more tender cuts, like fillet, the temperatures will make them mushy.
Preheat the oven to 375 deg F/190 deg C/Gas mark 5. Choose a cut of beef with a layer of fat on top: this provides basting juices to keep the joint moist. You'll need about 1.6kg/3.5lbs of boned meet, or 2.3kg/ 5lbs of meat on the bone to feed 6 people. Dust the surface of the beef with a level teaspoon of mustard powder and another of plain flour and run it in gently--this will make the fat extra crusty. Season with salt and pepper.
Place the roast directly on an oven rack just above the centre of the oven, fatty side up, with a drip pan on a rack beneath the roasting rack. This creates convection in the oven so that you do not need to turn the roast. The roast is placed fatty side up so that as the fat melts it will soak the rest of the roast in its juices.
While the beef is cooking lift it out of the oven every now and then (at least three times) and baste it with its own juices from the drip pan. This keeps the meat moist and makes sure it retails its flavour. Keep the oven door closed while you're basting so you don't lose heat.
Lower the temperature to 230 F/110 C after the first half hour. Roast at this temperature for another 2 to 3 hours, depending on if you want your meat rare or well-done. When the beef is dripping and brown on the outside, check the temperature with a meat thermometer. When the temperature of the beef is 135-140 F/57-60C, remove it from the oven. If you don't have a thermometer, insert a thin skewer into the thickest part of the joint. If the juice that escapes is red, it's still rare; if it's clear, it's well-done.
Let the roast beef sit on a heat-proof surface for 15 minutes to an hour, wrapped in tin-foil, before carving to serve. This will keep it warm while allowing the juices to settle, and as the meat relaxes, it will becomes easier to carve. Accompany your roast beef with gravy and vegetables. Potatoes, parsnips, horseradish and onions are popular options--as is Yorkshire pudding.