Working for a major UK air carrier, doing the same job as my colleagues who have 10 days off per?

... month, except in February when they get 9 instead of 10,
I am only entitled to 9 days off per month, except four times a year when I have 8 days instead of 9.
My contract T&Cs is not exactly the same as other senior crew as it's the result of takeover negotiations between present airline and union reps of former one, who acknowledged not spotting the issue during the deal. We had to sign the contract as it was presented in order to keep our job. Is this legal?