"The true principle of law is that a person is liable for his engagements (as for his torts) even though he is acting for another, unless he can show that by the law of agency he is held to have expressly or impliedly negatived his personal liability". 56 The circumstances in which that may be done were the subject of a classic analysis by Donaldson J in Teheran-Europe v ST Bolton Tractors[1968] 2QB 53 , at pages 59-60. He pointed out that "