As a general rule of Scottish law, extrinsic evidence of the parties’ intention as to whether or not they intended to be bound by obligations which they have entered into in writing is inadmissible. There may however be exceptional cases.
For a plea of error to succeed, it had to be shown that there was uninduced unilateral error going to the ‘substantials’ of the contract.
A party must take his contract as bearing whatever meaning the court will assign to it when it is called upon to interpret it. He is bound ‘by the interpretation which a court of law may put upon the language of the instrument.’
Judges: Lord Watson
References:  UKHL 1, (1890) 17 R (HL) 25, (1890) LR 15 App Cas 108
- Percy -v- Church of Scotland Board of National Mission, HL, Cited, (Bailii,  UKHL 73, House of Lords, Times 16-Dec-05,  2 WLR 353,  ICR 134,  IRLR 195,  2 AC 28, 2006 SLT 11,  4 All ER 1354)