My employers keep changing the terms of my employment contract. For example, six months ago they changed our weekly payment to monthly, and now they’re changing our holiday year to run from January 1st instead of April 1st, so it seems we only get four weeks holiday this year.
Your employers can’t change your employment contract without your agreement. But assuming you were given notice of the change, a tribunal may decide that changing your method of payment was not unreasonable. Taking a weeks holiday off you would be unreasonable (not to say illegal, since you have a statutory entitlement to a minimum of 5.6 weeks holiday a year). But it sounds as though you will still get 5.6 weeks holiday within the new 12-month period.