Heat the oil in a saucepan over a medium heat, add the flour followed by the mustard powder, salt and pepper. Begin whisking in the milk slowly to form a smooth sauce. Bring to a simmer and cook for 2 minutes, whisking continuously. Remove from the heat and set aside.
In a mixing bowl, beat together the eggs, vinegar and sugar. Add the egg mixture gradually to the milky sauce, whisking well after each addition. Return to a low heat and cook gently for about 10 minutes, stirring continuously, until the mixture coats the back of a spoon. Taste and adjust seasoning if desired.
Pour the salad dressing into sterilised bottles and store in the fridge between use.