In Great Hill Equity Partners ("G") II LLP v Novator One & Others ("N") the High Court held that the general rule that negotiations are only an aid to construction of the true meaning of the parties remains true and are generally not admissible.
What this means is that such evidence may be admissible only rarely and even then will not necessarily be conclusive.
In this case the meaning of agreement under dispute was a share option agreement but the principle applies to any contract in fact.
Issues frequently arise either because a contract is never signed or because there is disagreement over what was actually agreed.