HMRC have updated their guidance What will happen if you do not pay your tax bill.
An agency has been added to the list of debt collection agencies that HMRC may use to settle a debt.
https://www.gov.uk/guidance/what-will-happen-if-you-do-not-pay-your-tax-bill
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Income Tax is a substantial single-volume guide to all the income tax issues commonly faced by most accountants and tax practitioners.