Preheat the oven to 230°C/ 210°C fan/ gas mark 8. Put the beef on a rack in a roasting tin. In a small bowl, mix the oil and mustard together. Rub it over the joint, then season with freshly ground black pepper.
Cook the beef for 15 minutes per 450g, plus an extra 20 minutes (for well done, cook for 20 minutes per 450g, plus an extra 20 minutes). Roast the joint for 20 minutes, then reduce the temperature to 200ºC/ 180°C fan/ gas mark 6 and cook for the remaining calculated time.
Meanwhile, make the Yorkshire pudding batter by sifting 125g of the flour into a bowl. Create a well in the middle, then add the egg and half the milk. Whisk until smooth, then whisk in the remaining milk and 75ml cold water. Set aside while the meat finishes cooking.
When the meat is done, remove from the oven and drain off all but 2 large spoonfuls of the meat juices. Reserve for making the gravy. Cover the joint with foil and leave to rest while you make the Yorkshire puddings and gravy.
To make the Yorkshires, turn the oven up to 220°C/ 200°C fan/ gas mark 7. Add 1 tsp of the vegetable oil into each hole of a 12-hole muffin tin, then place in the oven to heat up for a few minutes. Remove from the oven and half fill each hole with batter, taking care with the hot oil. Cook the puddings for 20 minutes, until well risen, crisp and golden.
To make the gravy, skim the fat off the reserved juices and discard. Pour the juices into a small pan and bring to the boil. Blend the remaining flour with 2 tbsp water and stir in. Cook for 2-3 minutes, then add the beef stock. Simmer the gravy gently for a further 2-3 minutes, then strain. Carve the roast beef into slices and serve with the Yorkshire puddings and gravy.