In section 148(2) of the [1992 c. 5.] Social Security Administration Act 1992 (revaluation of earnings factors), for the words from “place” to the end there shall be substituted “place—
(a)since the end of the period taken into account for the last review under this section, or
(b)since such other date (whether earlier or later) as he may determine;
and for the purposes of any such review the Secretary of State shall estimate the general level of earnings in such manner as he thinks fit.”