Cheesy cheese sauce

Ingredients 
1 pint (600 ml) of milk 
1½ oz (40 g) of plain flour 
1½ oz (40 g) of butter 
2 oz (55 g) of grated mature Cheddar cheese 
1 oz (30 g) of finely grated Parmesan cheese 
pinch of cayenne pepper 
salt and pepper 
Method 
Melt the butter over a gentle heat in a medium sized saucepan. 
Add the flour and mix together to form a smooth paste and cook for 1 or 2 minutes. 
Very slowly, add the milk, a little at a time, whilst continuously whisking or stirring with a wooden spoon. 
Continue to stir or whisk and cook over a medium - high heat until the sauce starts to thicken and then reduce the heat and simmer for about 5 minutes. Do not allow the sauce to boil. 
Add the Cheddar and Parmesan cheeses and stir until the cheeses melt. 
Add the cayenne pepper and then season with salt and freshly milled black pepper, cook gently for a few more minutes. 
Stir well and then serve.