Oh I see what you mean now - I read it the wrong way.
Very little you can do. Most employment protection rights don't kick in until 12 months continuous service has occurred. The main exception is for 'automatically unfair reasons' for dismissal - these are listed here, so you can go through them and see if any of them could possible to stretched to apply to you.
http://www.businessli...793717&type=RESOURCES
Regarding changing your salary, this is a change in your contract so the empl,oyer has to give you notice. For you, the period of notice required in writing will be one week, unless longer because it says so in documentation (job offer letter?) you've received.