113.—(1) The Inland Revenue and the employer may, by agreement and consistently with the provisions of this Part, vary the terms of a PSA entered into by them.
(2) The agreement must be—
(a)in writing, and
(b)signed and dated by the employer and by the Inland Revenue.
(3) The last date for variation of a PSA is 6th July following the end of the tax year to which it relates.