First, preheat the oven to 220°C. Calculate the meat's cooking time, allowing 10–12 minutes per 500 g for rare meat, 15 minutes per 500 g for medium rare, and 20 minutes per 500 g for well done, all plus an extra 10–12 minutes. Put the beef, fat side up, in a large roasting pan and rub it all over with the oil. Mix the salt, pepper and mustard and rub evenly over the meat. Roast the meat in the centre of the oven for 20 minutes, reduce the temperature to 190°C and continue to cook for the remaining calculated time.
When the meat has been cooking for about 20 minutes, halve or quarter the potatoes, depending on their size, and place in a large saucepan. Cover with cold water, add the salt and bring to the boil. Drain well, return to the pan and shake over high heat for 1–2 minutes.
Remove the meat pan from the oven, spoon the fat from the bottom into a second pan and return the beef to the oven. If necessary, add extra lard or vegetable oil to the second pan to a depth of 5 mm. Heat on the shelf above the meat until the fat sizzles. Carefully add the potatoes, basting until well coated. Roast for about 1 1/4 hours, turning and basting them every 20 minutes, until they are crisp and golden.
While the meat is roasting, make the Yorkshire pudding batter. Sift the flour and salt into a bowl, make a well in the centre and put the eggs and a third of the milk into the well. Stir with a wire whisk, gradually incorporating the flour from the sides to make a thick batter, then beat until very smooth. Gradually whisk in the rest of the milk. Cover the bowl and leave to stand for 1 hour.
To make the horseradish sauce, put the wine vinegar, mustard, some salt and pepper and the horseradish into a small bowl and mix well. Whisk the cream until it holds a soft peak, then fold in the horseradish mixture, taking care not to overmix the sauce. Cover and refrigerate until required.
When the beef is cooked, transfer it to a heated serving dish, cover loosely with foil and allow it to rest while you cook the Yorkshire pudding.
If the potatoes are cooked, remove them from the oven and put them on a heated serving dish. Cover and keep hot. Otherwise, leave them to roast for a little longer while the oven heats up for the Yorkshire pudding.
For the Yorkshire pudding, use a 12-hole patty pan. Increase the oven temperature to 220°C. Take 3 tablespoons of fat from the meat pan and distribute it among the 12 patty holes. Place the patty pan in the centre of the oven and heat until the fat sizzles. Stir the Yorkshire pudding batter, distribute it evenly among the 12 patty holes and then cook for 10 minutes, or until puffed up and golden. (Alternatively, you can make the pudding in a 25 x 28 x 4 cm baking pan, cook it for about 20 minutes and then cut it into pieces.)
Meanwhile, make the gravy. Skim all but 2 tablespoons of fat from the meat pan. Stir in the flour and gradually add the stock and wine. Bring to the boil, stirring and scraping up the residue from the bottom of the pan. Strain into a saucepan and simmer, skimming off any fat that rises to the surface, for 5–10 minutes, or until slightly thickened. Season and pour into a heated gravy boat.
Slice and serve the beef with portions of Yorkshire pudding, gravy, roast potatoes and horseradish sauce, thinned, if necessary, with a little pouring cream or milk. Pass freshly made English mustard separately.