(1)A periodic standard contract may not be varied except—
(a)in accordance with sections 123 to 126, or
(b)by or as a result of an enactment.
(2)A variation of a periodic standard contract (other than by or as a result of an enactment) must be in accordance with section 127.
(3)This section is a fundamental provision which is incorporated as a term of all periodic standard contracts; section 20 provides that subsections (1)(b) and (2) of this section—
(a)must be incorporated, and
(b)must not be incorporated with modifications.