It all boils down to your terms of employment. Now these don't necessarily have to be in writing, although, from an evidential point of view, it helps. Terms of employment can be deduced from how you have done your job over a period of time.
If your employer tries to unilaterally change your terms of employment, he is, in effect, in breach of contract. Unless you agree to the change. If you don't accept the change, you need to let them know in writing as soon as possible as the absence of any written objection can be taken as tacit acceptance.
So, what you need to look at is - what were my original terms of employment, how has he changed them, am I happy with the change, what outcome do I want? If you want to keep this job, then your options may be more limited in that, taking action through an employment tribunal is unlikely to see you in that job in the future. If you aren't particularly bothered about keeping this particular job, then you have more flexibility in how you deal with the situation.
It is important that you send your employer a letter setting out whether you accept the change in your terms or not. There are new grievance procedures which came into force on 1st October 2004 and, if you don't follow them, you will fail at an employment tribunal (or at least have any award reduced to reflect this).
Keep detailed notes of everything which you feel impacts on your job and take any action swiftly to avoid the appearance of tacit acceptance.