Entire Agreement clause can still be open to extrinsic evidence if words / terms are unconventional

The Court of Appeal ruled that where terms used in a contract are ‘unconventional’ (ie, it is not possible to give them a meaning without referring to extrinsic evidence), it is lawful and ‘indeed, vital’ to refer to that extrinsic evidence to help define those terms; and the presence of an entire agreement clause in the relevant contract did not prevent this from happening. The extrinsic evidence did not add to or alter the words used in the main contract – it just explained what they meant.

It was therefore lawful to look at extrinsic evidence to help determine what they were intended to mean. A separate agreement comprised such extrinsic evidence and could therefore be referred to help determine what those words meant.

NHS Commissioning Board v Vasant and others [2019] EWCA Civ 1245 https://www.icaew.com/library/subject-gateways/law/legal-alert/2019-10/case-law-court-clarifies-when-extrinsic-evidence-can-be-used-to-explain-meaning