"If a figure were to be selected in the field of employment it would be likely to vary according to the context and in particular as between a case where the requirement or condition is applied on a national scale in respect of which reliable supporting statistics are available and those where it is applied in relation to a small firm or an unbalanced workforce where the decision may have to be made on far less certain evidence and to a large degree upon the basis of the Tribunal's own experience and assessment applied to such figures as are available"