4.—In section 3A (1) (inserted by the Act of 1959) of the Act of 1929, as amended by section 4 of the Act 1964—
(a) a reference to £450 shall be substituted for the reference to £300 where the work commenced on or after the 1st day of January, 1973, and before the 1st day of November, 1977, and
(b) a reference to £600 shall be substituted for the reference to £450 (inserted by this section) where the work commenced on or after the 1st day of November, 1977, and before the passing of this Act or commences after such passing.
