Where:
(a) a person is entitled to receive payments of a kind mentioned in section 54L (in this section called pre - assessment payments ); and
(b) the person is retired on the ground of invalidity with effect from a day occurring during the period of the person's entitlement to pre - assessment payments;
the person is entitled to invalidity pension in respect of a pension pay day immediately preceding a contribution day occurring during that period only to the extent (if any) by which the amount of pension that, but for this section, would be payable exceeds the amount of pre - assessment payment payable on that contribution day.