I really wouldn't fancy frozen mash potato myself if thats what you have made it with, but you can buy frozen fish pies at the supermarket so it must be ok.
Frozen mashed potato is fine as long as it wrapped well. If there are any breaks in the seal it will absorbe water (ice) and become water when defrosted. As a rule of thumb, if you cook a previously frozen raw product it is safe to refreeze after cooking.
I was told by a food expert that as long as the food is not hanging around too long you can re-freeze cooked or uncooked , bearing in mind some bacteria can reproduce in 20mins, so the longer it is hanging around the more likely it is to have extra bacteria than when it came out of the freezer. Also food should be defrosted naturally in the fridge not out on a worktop for the same reasons. Most bacteria will not survive the temperature of the fridge.