The company I work for have an internal website with a section for hr stuff. Under "policy and procedure" they list details of things like holidays and sickness etc. However since a new HR manager has been appointed things keep changing on it without the staff being formally informed of it....for example I just looked at the sickness terms this morning and after 1 years' service you were entitled to 6 weeks full pay, now it has been changed to 4 weeks. Surely this is a change of contract and the correct procedures should be followed?
This would be a change of terms and conditions, which can't just be implemented in that way, you are right.
See here about what your employer needs to do to vary terms and conditions, and here about what you should do if you don't agree.
The only way they could argue that this doesn't apply is if the terms and conditions they are changing are specified as non-contractual or subject to variation. Even then, it would be risky of them to try and vary them without any consultation. I suspect they are hoping no one has noticed!