Box Mac and Cheese

I feel a bit honoured to have joined the (I assume) elite club of those who have failed at making a packet cheese sauce for the mac and cheese.
The feeling I had when the sauce separated, into oil and some sort of hard orange gunk is one I will never forget - at least not very soon. I emptied the sauce sachets out of a couple of other boxes of mac and cheese in the cupboard, and ruined that lot too, just for good measure.
On consultation with a friend, I realised the pot had to be cool - ie, you shouldn't melt the butter for the sauce in the pot just before emptying in the sachets of powder.
Mr 7, who was helping with the cooking, has been gaily telling everyone about how we were meant to have mac and cheese, but ended up with pasta sauce instead (lucky we had that in the cupboard).