The legal advice I received and paid handsomely for in 2001 (concerning reduction of pay under pay 2000) was this:
If your employer fundamentally changes your terms of conditions or employment, then you have a period of three months maximum to register your complaint. If you go outside the three months then you are deemed to have accepted the changes.
If you decide not to accept the said changes then you are ENTITLED to leave under the normal notice period that the law allows. For those of you in HM Armed Farces that equates to between 3 months and 6 months depending on your circumstances. MOD and PMA can say what they like to contradict it, and they will but it is the law.
I eventually left for different reasons, was quoted 12 months by PMA as a PVR waiting time and was out in less than 5 months!! If you have another job to go to then they have to prove their operational reasons for keeping you on.
I suggest any reputable solicitor specialising in employment law would be a good starting point, but they do not come cheap and the MOD/PMA believe they are above the law of the land..
