So -got home from work at hald 7 yesterday and started to get dinner ready for us and the PIL. MIL came into the kitchen to tell me that they didn't need any food as they had just had a sandwich.
I had bought stuff that had to be prepared then - ie it was not freezable. Them not eating their portions means it will go to waste.
I got quite cross with her and eventually she did apologise 'for the misunderstanding' which I find irritating. There was no minsunderstanding, they are staying at my house and chose to not follow the convention of eating what your host prepares when they prepare it.
The resolution they suggested is that we now need to discuss who's eating what and when every morning. Not ideal as I food shop on the internet so plan menus in advance, but apparently this is the solution.