We are recruiting a woman to do the job of a man who is retiring. Do we have to pay her what he is getting?
No, there is no requirement to pay the same as the predecessor, however you must be able to justify why you are doing so.
There must be proof that the lower pay is for reasons other than gender, rather than for her to prove that it is because of her sex. Examples of material difference include; the predecessor has more experience, the predecessor has higher qualifications.
If you are unable to provide substantial evidence then you face being judged under the Equality Act.
However, most claims of equal pay require the claimant to compare herself with another current employee. Claims cannot generally be made based on using a successor as a comparator.
More information on gender pay discrimination can be found at: equal pay faqs