EIM75020 advises "If a pension provider discovers a long-standing underpayment of pension, the underpayment is calculated and paid in a single sum.
Where the provider is required to operate PAYE, they operate it on the lump sum arising which may give rise to higher rate liability for a pensioner who is usually a basic rate taxpayer.
In this situation, the pensioner should contact HMRC at the end of the tax year in which the arrears were paid and supply a schedule showing the years to which underpayments are attributable (on the accrual basis), asking for the payments to be related back to the relevant years.
HMRC will spread the payments back over the relevant years and recalculate liability.
Underpayments in the earlier years may be set-off against the resulting overpayment in the year of the lump sum payment.
The address to write to is
HM Revenue and Customs
EIM75020 - The taxation of pension income: pension payments made in arrears or in advance