No, but if you pay her less, you need to be able to justify the difference. It is up to you to prove that she is getting less for reasons other than her sex, rather than up to her to prove that her sex is the reason. If you cannot prove that there are other, adequate reasons for the difference, you will be judged to be in breach of the Equal Pay Act.
You will have to establish that the difference in pay is genuinely due to a 'material difference' other than sex. Such a material difference might include, for instance, the fact that her predecessor has much more experience, or higher qualifications, than she has. It would not be acceptable to pay her less because, as a woman, she was prepared to accept less pay, or because women in general are prepared to accept less pay.
In most cases an employee who wants to make an equal pay claim must compare himself (or herself) against another employee who is working for you. However, when the comparison is with a predecessor, it does not matter if the predecessor has now left your employment. In a recent case, an Employment Tribunal has overturned previous decisions and said, however, that former employees may not compare the package they had been getting with that awarded to their successors.
