"In labour-intensive sectors, a group of workers... may constitute an economic entity and such an entity is capable of maintaining its identity after it has been transferred where the new employer does not merely pursue the activity in question but also takes over a major part, in terms of their numbers and skills, of the employees specially assigned to his predecessor to that task. In those circumstances the new employer takes over a body of asset[s] enabling him to carry on activities of the transfer or undertaking on a regular basis."