Cheese sauce separating?

[Mummysaysno]

Am I right in thinking the reason why a cheese sauce separates is if the cheese is added to the thickened butter/flour/milk sauce while the pan is still on the heat?
Is there any other reason why a sauce would separate?
Thanks!!

If you just heat it very gently after adding the cheese, it should be ok. However, you're right that it's safer to add the cheese off the heat to the white sauce.

If you use cornflour instead of ordinary flour then cheese sauce is almost always 'rescuable' and yes, add the cheese once you've taken it off the heat.