If it is a P21 you should check with Revenue as to why the umderpayment is so large - was it due to your employer not operating the tax credit cert correctly, did you claim a credit that they now believe is not due to you, was there a "Week 53" in the taxyear?
If it was a self-assessment Tax Return did you claim everything you are entitled to, even if it was already included on your tax credit slip?