I14Protection of third party's entitlement from retroactive change
1
No account is to be taken of the cancellation or modification of an undertaking contained in a contract where and in so far as the undertaking is being enforced or otherwise invoked—
a
by virtue of a person's third-party right to do so, and
b
in consequence of something happening or not happening prior to the undertaking being cancelled or (as the case may be) the modification being made.
2
Subsection (1) does not apply in relation to a cancellation or modification if the contract provided that it may be made with retroactive effect.