TTOG6605 | Case closure: reopening settlements: reasons
From HM Revenue & Customs · Technical Teams Operational Guidance
There are four main reasons why settlements may need to be re-considered:
the taxpayer may seek to re-open on the grounds that the settlement was harsh or excessive
the taxpayer may not dispute the settlement but may claim inability to pay
SI may wish to re-open on the case that the settlement was inadequate or flawed so that it is impossible to enforce collection
there may be a carry back or similar claim (see TTOG6625).