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HMRC internal manual

VAT Default Interest Manual

Circumstances affecting default interest: Long first period returns

Where a manual central assessment (VAT152A) is issued because a long first period return is not received on time, any interest due is calculated as usual from the reckonable date. 

The reckonable date, see VDIM2030, is the due date of the return. The interest is therefore only calculated from the due date despite the fact the assessment may include arrears for example that are two years old.