Where any of the claimant’s income is income the income tax on which (at the basic rate) he is entitled to charge against any other person, or to deduct, retain or satisfy out of any payment, he shall not be entitled to relief under this Chapter in respect of that income, except to the extent, if any, that the relief would exceed tax at the basic rate on that income.

In subsection (1) above the references to relief under this Chapter do not include references to relief consisting in such an income tax reduction as is mentioned in section 256(2).

Notwithstanding subsection (1) above, relief under section 273 may be given to the extent that the deduction from tax provided for by that section can be made from so much of the income tax with which the claimant is chargeable as exceeds what would be the amount of that tax if all income tax were chargeable at the basic rate to the exclusion of any other rate.