[Thanks to Ben Cooper of Old Square Chambers, junior counsel for Mrs Wilson, for providing this case summary]
The Court of Appeal's decision in Wilson v Health & Safety Executive is authority for the proposition that, in an equal pay claim arising from a service-related pay progression scheme which has a disparate impact on women compared with men, it is open to an employee to challenge both the adoption of length of service as a determinant of pay and the particular way in which it is used or applied.
In order to be allowed to pursue such a case, the employee is required to do no more than show that her claim has some prospect of success in the sense that there is evidence from which, if established at trial, it could properly be found that the adoption or use of length of service is not justified and/or is disproportionate.
The legal burden of proof in relation to objective justification, including showing proportionality, remains on the employer in accordance with ordinary principles under the Equal Pay Act 1970 (read together with the Sex Discrimination Act 1975). That approach is consistent with Community law under Cadman v HSE  ICR 1623, ECJ. However, if Community law were less favourable to employees in that regard, domestic law would prevail because Community law sets only minimum guarantees.