"A basic working test, it seems to me, is to ask whether, if that part of the business had been separately owned before the transfer, the worker would have been employed by the owners of that part, or by the owners of the remaining part. The only exception I would admit to the requirement that an employee must be 'wholly' engaged in that part of the business would be where an employee was required to perform other duties to an extent which could fairly be described as de minimis . On the other hand, if a worker in fact is engaged in the activities of the whole business or in several parts then he cannot be regarded for the purpose of the directive as an employee 'of' the part of the business transferred."