Associate
Might be worth you looking up the term "custom and practice" on this...Well to add to the joy of working on Christmas day we have just been told that we are not even getting paid double wage like we have every other year, which made it a little easier.
http://www.direct.gov.uk/en/Employment/Employees/EmploymentContractsAndConditions/DG_10037109
Terms implied by custom and practice
These are specific to an employer or kind of work. They are arrangements that have never been clearly agreed but over time have become part of the contract.
For example, you might get a Christmas bonus every year, or the business might close early on particular days.
If a company practice has become a part of your contract then your employer must stick to it, and cannot normally change it without your agreement.
Worth a shot to see if you can get a sweetner?