For the Roast Rib of Beef do you set the roast in the pan just as it looks in the picture? Rib bones standing?
For Nigella's recipe the rib of beef is put in the roasting pan in an upright position (as shown in the photograph that accompanies the recipe), with the bones pointing upwards. This helps the top layer of fat to drip down as it melts and keep the meat moist during cooking. In the US it is known as a "standing rib roast". If you find it difficult to get the beef to sit in this postion on its own then you can wedge and position slices of onion to help keep the beef in place. Before you preheat the oven make sure that you have enough room to fit the upright joint in and lower the level of the oven shelf as necessary.