The practice of not being allowed to carry over holiday pay into the new leave year is pretty standard - mainly for administrative reasons and your employer is within their legal rights to do this. And as unfair as it seems you may not see it again.
However, the issue at hand seems to be what you were told the cut off date was - if it was written on your payslip then thats quite clear. However, if your manager told you something different then you could say that you went on what you were told by your manager and losing pay because of their inaccurate information is hardly fair.
Good luck.