If so, the Equal Pay Act of 1970 (perhaps the oldest employment legislation on the books?), may mean you need to repay the difference over the preceding 6 years.
This is the case even if the jobs are completely different, but have equal value. For instance, a comparison has been allowed between a catering trainee and an apprenticed painter.
A useful guide and spreadsheet has recently been published by the Equality and Human Rights Commission and British Chambers of Commerce, aimed at helping SMEs find out whether they have an unlawful pay gap or not.
Following the guidance and filling in the spreadsheet accurately will be a great assistance if a claim is brought. This should not take long. But you will see that it is very basic. Perhaps too basic. Only time and future litigation will tell.