The Double Taxation Relief (Taxes on Income) (General) Regulations 1970

9.  If it is discovered after a notice has been given under Regulation 2(2) that the non-resident is not entitled to exemption or partial relief from tax in respect of income referred to in the notice, any tax which, but for the notice, would have been deductible from any payment made to the non-resident by the United Kingdom payer but in compliance with the notice has not been so deducted—

(a) may be assessed on the non-resident under Case VI of Schedule D by an Inspector, or

(b) shall, if a direction to that effect is given by or on behalf of the Board, be deducted by the United Kingdom payer out of so much of the first payment made to the non-resident after the date of the direction as remains after the deduction of any tax deductible therefrom under the Income Tax Acts, and any balance which cannot be deducted out of the first such payment shall be deducted, subject to the same limitation, out of the next such payment. and so on until the whole of the tax (the amount of which shall be specified in the direction) has been deducted.

Any tax which the United Kingdom payer is required to deduct under paragraph (b) of this Regulation shall be accounted for as if it was tax deductible under section 53 of the Income and Corporation Taxes Act 1970 in respect of the payment from which it is deducted.