1. Melt the margarine in a saucepan over a medium heat but don't let it burn.

2. Add the flour and stir until fully mixed together into a thick paste - this forms the "roux". Cook the roux for 1 minute.

3. Gradually add the milk, stirring until fully combined with each addition.

4. When all the milk has been added stir continuously over a medium heat until the sauce begins to thicken.

5. Cook over a high heat until the sauce starts to boil, then reduce the heat and simmer gently for 5 minutes stirring continuously. The sauce should be smooth and glossy without any lumps.

6. Season with black pepper.