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HMRC internal manual

COTAX Manual

Interest: how interest is calculated: clerical calculation of late payment interest (Action Guide menu)

Reference Action Guide
COM80035 Late payment interest: clerical charge (CT Unit Cumbernauld)
COM80036 Late payment interest: clerical charge (debt management)
COM80037 Interest on carry-back / S458 CTA 2010 (formerly S419(4) ICTA 1988) relief