Method

Pop the flour into a bowl and make a well in the center
Break the egg into the well, beat and gradually incorporate the flour, then the milk, followed by water, mustard, salt and pepper
Add the oil to a Yorkshire pudding tin and place in a hot (220C) oven, get sizzling hot (10 minutes)
Pour the batter into the sizzling hot fat.
Return to the oven immediately.
The pudding will take 20-30 minutes to rise and become crisp and golden; if they are still limp, pop them back into the for another few minutes.
Serve as soon as possible: if it has to wait around too long it loses it’s crunchiness, but you can just flash it through the oven to crisp up again.