122(1)
A periodic standard contract may not be varied except—
(a) in accordance with sections 123 to 125, or
(b) by or as a result of an enactment.
122(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.
122(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.