Measure the flour, mustard, sugar and salt into a saucepan and mix together. Gradually stir in the milk and whisk well to make sure there arc no lumps.
Place the pan over the heat and bring slowly to the boil, stirring all the time. As the mixture begins to thicken in the pan, remove from the heat and beat very thoroughly – this helps to make a smooth mixture.
Replace over the heat and allow to come up to the boil. Stir in the vinegar and add the butter cut in pieces. Continue to stir over the heat until the butter has melted and the mixture is blended. Remove from the heat and cool for a few minutes.
Lightly mix the egg in a basin. Whisk in a little of the sauce. Blend well and pour the mixture back into the saucepan. Stir over the heat just for a few moments to cook the egg. Pour the salad cream into wide-necked bottles and seal. Leave until cold. Shake the bottle before using.