Keep macaroni cheese creamy after refrigerating
Having made this dish again recently and experimenting with it based on the comments here can report the following:

A good creamy texture in the pot after the initial preparation will become a single hard lump in the fridge. Adding more milk before refrigeration will:


Prevent it from solidifying
Reheat (e.g. in a microwave) to a close enough texture to the original


The amount of milk that needs to be added is about the same quantity as went into the dish originally, i.e. if you used 1 litre in the dish before serving it (assuming only a small quantity removed) add another 1 litre of milk before refrigerating. If you served half of it, only add half a litre of milk, etc. The only downside is that the extra liquid separates slightly in the fridge, leaving watery pools on the surface of the food. This extra moisture can simply be stirred back into the dish before reheating.