Once you know what they want to try to change, then the directgov site is a good starting point. as it will tell you, what they can, and cannot get away with.
http://www.direct.gov.uk/en/Employment/index.htm
Also ACAS are pretty good with plain information.
http://www.acas.org.uk/index.aspx?articleid=1461
I phoned them last year, when our company wanted to change our pay dates from the end of the month, to the middle of the next month! The advisors where extremly helpful.
Hope this is of some help.