(1)For the purposes of sections 51 and 52, the comparable tax liability of an individual is the amount of the individual's tax left after Step 6 of the calculation in section 23 for the tax year, making that calculation with the modifications set out in subsections (2) and (3).
(2)In making that calculation, do not deduct any tax reduction under—
(a)section 788 of ICTA (double taxation arrangements: relief by agreement), or
(b)section 790(1) of ICTA (relief for foreign tax where there are no double taxation arrangements).
(3)If the individual's entitlement to a tax reduction under this Chapter is extinguished under section 423(4) (gift aid: restriction of reliefs) to any extent, deduct from the amount calculated in accordance with subsections (1) and (2) the amount by which the tax reduction is reduced.
(4)A notice under section 51 or 52—
(a)must be given on or before the fifth anniversary of the normal self-assessment filing date for the tax year to which it relates,
(b)must be in the form specified by the Commissioners for Her Majesty's Revenue and Customs, and
(c)cannot be withdrawn.
(5)For the purposes of this section a person is treated as being entitled to a tax reduction under section 788 of ICTA if the person is entitled to credit against income tax under double taxation arrangements.