Put the beef in a roasting tin, without adding fat. Sprinkle with pepper. Roast in a hot oven at 220 C / 425 F for 15 minutes. Lower the heat to 190 C / 375 F for the rest of the cooking time, allowing 15 minutes per 500g / 1lb. Baste the mean frequently with pan juices. The beef should be served underdone and left to rest 15 minutes before carving.
To make the Yorkshire pudding, sieve the flour and the salt together in a mixing bowl. Add the eggs and stir. Add half the milk slowly and stir until the mixture is smooth. Add the remaining milk slowly and beat the batter well, then beat in the cold water. About 15 minutes before the beef is ready, pour 2 tbsp of its fat into a 25cm / 10" square tin. Put the tin in the oven, and when the fat is sizzling hot, pour in the pudding batter and bake it at the top of the oven for 30 minutes. (The meat should be removed 15 minutes earlier to rest before carving). The pudding should be well risen, puffy, crisp and brown on the top and bottom, and should be served cut in square straight from the in in which it was baked.
To make the horseradish sauce, whip the cream until it forms stiff peaks, and fold in the horseradish, sugar, mustard, salt, pepper and vinegar. Serve cold.