Yes. I do this quite often when I have a small amount left from one meal. I think the secret is not to make it too wet when mashing with milk and then to give it a stir-through after re-heating in the microwave (having defrosted beforehand)
Hope this helps.
Holmesy - I take those little bits of leftovers and put them in a plastic container - freeze them. Then when you need a quick lunch or dinner for one and don't want to cook - straight from the freezer to the microwave. Much better than just a sandwich:)