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HMRC internal manual

VAT Default Interest Manual

Circumstances affecting default interest: Bulk period assessments

An Officer’s Assessment raised in an evasion case may include periods more than 4 years old which are processed as a bulk assessment (period 00/00), however the computer does not recognise period 00/00 and so cannot calculate interest for this period. 

You will need to increase the amount of tax liable to interest through the VAT663 procedure provided another under declaration has been processed on the same VAT641 / VAT642, see VDIM6000.