Ah! Thats why it didn't work.
The problem is the the page had two checkboxes, one for approving an entry, and one when that action has been completed.
These two boxes sent different emails if the boxes were ticked, however if I completed the task after approving it, the second update would send both emails as both boxes would be ticked, hence my reason for trying to create a rule that would send one email if one box ticked and one box clear, and the next email if both boxes were ticked.