HMRC cocked up my tax codes too, resulting in an underpayment of income tax (PAYE) on my RAF, Private and State Pensions over the past two years.
This draft letter, which I think was compiled by the Institute of Accountants and published in one of the Sunday Papers is a good starter.
Dear Sir,
I am in receipt of the P800 tax calculation for the (2008/09 and 2009/10 tax year(s))
The calculations indicate that I have a further tax liability. As income tax is deducted at source from my income under PAYE and you were in possession of all the relevant facts regarding my income, I had been unaware that my tax affairs were not in order.
I therefore wish to apply under the provisions of Extra Statutory Concession A19 that the underpayment of tax now being demanded should be written off, on the following grounds:
Either the demand for further tax was issued more then 12 months from the end of the tax year in which HMRC was provided with the relevant information to calculate my tax position correctly:
or HMRC have allowed the arrears to build up over more than one tax year, despite all relevant information being provided to you:
or HMRC have failed more than once to make use of the relevant information provided to you about my income.
I look forward to receiving your confirmation that the further tax liability will be written off accordingly.
Your faithfully
http://www.hmrc.gov.uk/specialist/esc.pdf