(1)A secure contract may not be varied except—
(a)in accordance with sections 104 to 107, or
(b)by or as a result of an enactment.
(2)A variation of a secure contract (other than by or as a result of any enactment) must be in accordance with section 108.
(3)This section is a fundamental provision which is incorporated as a term of all secure 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.