Mix flour and salt together until blended. Make a well in the flour, add the milk, and whisk until consistent. Beat the eggs into the batter. Add water and beat again until the mixture is light and frothy. Set aside for an hour (or, if it's the day before, cover in the fridge overnight).
If the batter has been refrigerated, allow it to come up to room temperature before using. When the roast beef is ready to come out of the oven, ready the mixture.
Preheat oven to 400 degrees F (205 degrees C).
Pour off drippings from roast beef and measure out desired amount (about 1/2 cup should do). Pour drippings into a 9x12 inch baking dish and place into the oven until the drippings sizzle. Pour the batter over the drippings and bake for 30 minutes (or until the sides have risen and are golden brown). Cut into eight portions and serve immediately.