S2(1) where third party has rights under S1 parties cannot vary/extinguish contract to change third party entitlement, without consent, if the third party has communicated his assent to the promisor to the term (by words or conduct, if by post it is effective when received), promisor is aware the third party relied on the term, or the promisor can reasonable have forseen the third party to rely on it and the third party has relied on it (unless expressly stated in the contract, then they can vary/extinguish third party rights when they like (SS(3))
Don't need tird party consent where S2(4)(a) their whereabouts cannot be easily ascertained, S2(4)(b) they are mentally incapable of giving consent or S2(5) their reliance on the term cannot be reasonable ascertained