"…erred in directing itself that if an employer behaves unreasonably towards a black employee, it is not to be inferred, without more, that the reason for this is attributable to the employee's colour in that the employer might very well behave in a similarly unreasonable fashion to a white employee. Such hostility may justify an inference of racial bias if there is nothing else to explain it. Whether there was an explanation such as that posited by the Tribunal will depend not on a theoretical possibility, that the employer behaves equally badly to the employees of all races, but on evidence that he does."