If you have paid too much tax

R38 - Tax Claim

If you have paid too much tax under PAYE scheme and want to claim back the overpayment, use form R38. You can also use this form to authorise a representative to receive the payment on your behalf. Trust customers should use form R38(SA)

source: http://search2.hmrc.gov.uk/kb5/hmrc/forms/view.page?formid=816&record=wHfrcAhYKW4