The amount of tax charged by an assessment is a debt payable to HMRC. The date on which it is payable (i.e. the date from which HMRC may commence enforcement proceedings) depends on the date on which the taxpayer is ‘given’ notice of the assessment, rather than the date on which notice of the assessment is issued.
The word ‘given’ is the same as was examined by the Special Commissioner in Holly in the context of the receipt of enquiry notices. If the notice is given on day 1, the tax is due on day 30.