Lord Toulson said ,
1. Vicarious liability in tort requires, first, a relationship between the defendant and the wrongdoer, and secondly, a connection between that relationship and the wrongdoer’s act or default, such as to make it just that the defendant should be held legally responsible to the claimant for the consequences of the wrongdoer’s conduct.
In this case the wrongdoer was employed by the defendant, and so there is no issue about the first requirement.
The issue in the appeal is whether there was sufficient connection between the wrongdoer’s employment and his conduct towards the claimant to make the defendant legally responsible.
By contrast, the case of Cox v Ministry of Justice  UKSC 10, which was heard by the same division of the court at the same time, is concerned with the first requirement. The judgments are…intended to be complementary to each other in their legal analysis…
The present law
44. In the simplest terms, the court has to consider two matters. The first question is what functions or “field of activities” have been entrusted by the employer to the employee, or, in everyday language, what was the nature of his job. As has been emphasised in several cases, this question must be addressed broadly…
45. Secondly, the court must decide whether there was sufficient connection between the position in which he was employed and his wrongful conduct to make it right for the employer to be held liable under the principle of social justice which goes back to Holt. To try to measure the closeness of connection, as it were, on a scale of 1 to 10, would be a forlorn exercise and, what is more, it would miss the point. The cases in which the necessary connection has been found for Holt’s principle to be applied are cases in which the employee used or misused the position entrusted to him in a way which injured the third party….
46. Contrary to the primary submission advanced on the claimant’s behalf, I am not persuaded that there is anything wrong with the Lister approach as such. It has been affirmed many times and I do not see that the law would now be improved by a change of vocabulary. Indeed, the more the argument developed, the less clear it became whether the claimant was advocating a different approach as a matter of substance and, if so, what the difference of substance was.
The present case
47. In the present case it was K’s job to attend to customers and to respond to their inquiries. His conduct in answering the claimant’s request in a foul mouthed way and ordering him to leave was inexcusable but within the “field of activities” assigned to him. What happened thereafter was an unbroken sequence of events….
First, I do not consider that it is right to regard him as having metaphorically taken off his uniform the moment he stepped from behind the counter. He was following up on what he had said to the claimant. It was a seamless episode.
Secondly, when K followed the claimant back to his car and opened the front passenger door, he again told the claimant in threatening words that he was never to come back to petrol station. This was not something personal between them; it was an order to keep away from his employer’s premises, which he reinforced by violence.
In giving such an order he was purporting to act about his employer’s business. It was a gross abuse of his position, but it was in connection with the business in which he was employed to serve customers. His employers entrusted him with that position and it is just that as between them and the claimant, they should be held responsible for their employee’s abuse of it.
Lord Dyson said,
50. As Lord Toulson has explained, the test for holding an employer vicariously liable for the tort of his employee has troubled the courts for many years.
The “close connection” test (whether the employee’s tort is so closely connected with his employment that it would be just to hold the employer liable) was first articulated in this jurisdiction by the House of Lords in Lister v Hesley Hall Ltd  1 AC 215…. As Lord Nicholls said in Dubai Aluminium Co Ltd v Salaam  UKHL 48;  2 AC 366, para 26, the test is imprecise, but that is inevitable given the infinite range of circumstances where the issue of vicarious liability arises. The court, he said, has to make an evaluative judgment in each case, having regard to all the circumstances and to the assistance provided by previous court decisions on the facts of other cases.
54. It is true that the test is imprecise. But this is an area of the law in which, as Lord Nicholls said, imprecision is inevitable. To search for certainty and precision in vicarious liability is to undertake a quest for a chimaera.
Many aspects of the law of torts are inherently imprecise. For example, the imprecise concepts of fairness, justice and reasonableness are central to the law of negligence. The test for the existence of a duty of care is whether it is fair, just and reasonable to impose such a duty. The test for remoteness of loss is one of reasonable foreseeability.
Questions such as whether to impose a duty of care and whether loss is recoverable are not always easy to answer because they are imprecise. But these tests are now well established in our law. To adopt the words of Lord Nicholls, the court has to make an evaluative judgment in each case having regard to all the circumstances and having regard to the assistance provided by previous decisions on the facts of other cases.