If you have not paid enough tax, HMRC can claim it from you, usually within four years. In some circumstances (eg deliberate tax evasion) the time limit is 20 years. HMRC could charge penalties if you are at fault.

Arrears of tax may be wholly or partly waived if they have arisen because HMRC failed to take account of information they have about you and you reasonably believed your affairs were in order. This concession may be given if you are notified of the arrears after the end of the tax year following that in which HMRC received information indicating you had underpaid tax or if HMRC failed repeatedly and arrears have built up over two or more tax years. If you believe this applies to you, write to HMRC claiming a waiver under extra statutory concession A19. For more information see www.litrg.org.uk/tax-guides/employment/what-if-i-do-not-pay-enough-tax/extra-statutory-concession-a19