“There are many ways to categorise a group of employees. When deciding which categories it should list in the notice, the union should consider choosing a categorisation which relates to the nature of the employees’ work. For example, the appropriate categorisation might be based on the occupation, grade or pay grade of the employees concerned. The decision might also be informed by the categorisations of the employees typically used by the employer in his dealings with the union. The availability of data to the union is also a legitimate factor in determining the union’s choices”