After many tried and failed attempts I managed to successfully figure out how to freeze and then thaw out mashed potato without resulting in watery and icky mash.

The process is rather simple:

1. When you freeze the mash add a dollop of butter in when the mash is cool (so that it does not melt)
2. When you wish to thaw it out, put it on defrost in the microwave for approx 5 minutes
3. Take out and stir, repeat for 2 minutes until there are no remaining frozen bits
4. Even if the mash looks watery don't pour away any water 
5. Once the mash is thawed put it on for about 1-1.5 mins on high in the microwave, stirring every 30 seconds.

Once done you will have mash that is as good as before it was frozen and won't be watery.

Do you have any other tips?

Stuart