Hi Simon, dont think they can do that, depends on how long you have been there and your contract of employment.
Think it would be Unfair Dismissal, or Constructive Dismissal, anyhow if you have been with them over two years you have rights, that they can not just change, without compensation.
Think your brave to sign back up, as you know where you will most likely end up, think I would go the Tank route, least you have some heavy metal round you.
Best of luck with the next move.