HMRC internal manual

Self Assessment Manual

Returns: view and amend return: dealing with taxpayer amendments (Action Guide menu)

You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.

SAM124062 Amending trust returns (including repairs after 31 December) (Action Guide)
SAM124063 Repairs to amended returns after 31 December (individuals) (Action Guide)
SAM124064 Return amendments: bankruptcy cases (Action Guide)
SAM124066 Return amendments: computer calculation is incorrect (Action Guide)
SAM124067 Return amendments: computer cannot calculate liability (Action Guide)
SAM124068 Return amendments: last year SA return required signal already set (Action Guide)
SAM124069 Return amendments: pension scheme tax return (Action Guide)
SAM124070 Return amendments: prior year adjustments (individuals and trusts) (Action Guide)
SAM124071 Taxpayer amendments (individuals and partnerships): basic action (Action Guide)