[I]f the written language of the agreement has a 'plain meaning', evidence will not be admissible to contradict that meaning ... The purpose of this rule ... is to hold parties to bargains reduced to writing which are unambiguously expressed. There is a link here (naturally enough) to the rule for construction of unambiguous statutory language. If the purpose of that language is clear, the duty of the court is to give effect to the intention of parliament derived from that unambiguous language ... The social purpose secured by such rules is to discourage litigation, with the time consuming, costly and dilatory exploration of detailed facts and the resolution of conflicts of recollection and testimony. It is to discourage curial exploration of the unfathomable depths of subjective intentions. It is to add certainty by adherence to the effect of the clearly expressed written word.