Our basic pay is the same for men and women, but men get more overtime. On the other hand, women get maternity pay. Can we argue that they win as much as they lose?

No. Anyone making a claim under the Equal Pay Act is entitled to compare any term in her (or his) contract with the equivalent in the comparator's, and if the claim succeeds, to have the less favourable term improved to the level of the more favourable one.