The first thing to do is check the figures carefully. If they are wrong contact HMRC and ask for them to be corrected and see if that removes the underpayment. If you still have an underpayment or the figures are correct, ring Tax Help for advice. They will help you determine the reason for the problem and to see if there has been HMRC or employer error. They will be able to advice you on your rights as a taxpayer and the best way forward. Also see the link to LITRG on the left for further notes on this subject.
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