Must there be a comparator group?

Yes. You can only succeed with a claim for indirect discrimination if you can point to a group who are the same as you in all other material respects, but who are not disadvantaged by the practice. For example, in Mr Homer’s case, the comparator group would have been his colleagues who were not approaching retirement and who had enough time to obtain a law degree and secure the promotion (Homer v Chief Constable of West Yorkshire Police [2012] UKSC 15).