Employer is reducing my annual leave?

After 11 years service I have now been told that because of the new rule on including Bank Hols in my annual leave my holiday has reduced from 4 weeks 2 days (an extra day has been given for every 5 years service) to just under 4 weeks. This is because I work 9 hrs on Mon/Tue 5.5 on Wed/Thurs, which means 9hrs is deducted for every B Hol. I have never had a contract. Can they do this? Someone else who never works a Monday has ended up with the equivalent of 5 weeks holiday and they have only been there 2 years!