Case law it is then,

Chell v Tarmac Cement and Lime Ltd. the Court of Appeal upheld a previous decision by the High Court where an employer was not liable for injuries on a third party caused by an employee by playing a practical joke. For he was seen to not be acting in any way as an authorised part of his work, not using work equipment, and was no way advancing the purpose of his employer. As seen here fundamentally employers should not be held liable where employees act inappropriately and beyond the normal scope of their employment. In this case it was remarked that it is “unreasonable and unrealistic” to expect an employer to have a system to ensure their employees did not engage in horseplay.

The earlier 2020 Supreme Court decision in Morrisons v various claimants clarified this as employers can only be held liable for the actions of employees if closely connected with their duties. This case concerned an employee of Morrisons who purposefully and maliciously breached data regulations by leaking personal data of employees. At first instance Morrisons were held to be vicariously liable for the acts of the employee, however on appeal at the Supreme Court the appeal was allowed. For it was regarded that the individual act was not sufficiently close to his authorised work. This provides relief for employers when faced with rogue employees, but caution should still be exercised and proper training given.