If you first received the state pension in the year to 5 April 2011, HMRC forgot to include that pension income in your PAYE code for 2010/11. If this is the case you may not have paid enough tax for that tax year. HMRC will send you a PAYE reconciliations (form P800), which will set out how much tax remains underpaid for 2010/11.

If the tax due is less than £2,000, the amount will be automatically collected through your PAYE codes for the three years to 2014/15. Where the amount due is £2000 or more we can help you negotiate with HMRC to spread the payment over time.

Tax Faculty comment on pension PAYE errors

http://www.ion.icaew.com/TaxFaculty/21959