Just to make you aware, it is very very difficult to prove these cases where the role is professional/performance related. Much case law on this has been around very large employers where people do exactly the same work, day in, day out (eg factory line staff), or where they work to exactly the same terms and conditions and have same number of years experience and identical qualifications and where there are no performance factors to take into account (lots of classic case law around dinner ladies vs dustbin workers and suchlike). This doesn't mean that you shouldn't use or quote your rights under this legislation, just that the bar in proving such cases for professional individuals is much much higher, simply because there are so many variables at play.
One thing that helps you hugely but was not in your OP is that other women got a payrise whilst you were on maternity leave and you didn't. That bit in itself does look a bit damning for them. That's the bit I'd concentrate on if it were me, and I'd be pushing for a backdate to that point in time (presumably taking effect in practice as soon as you came back from ML)
Here's the stuff you need to mention discreetly in passing - it's a bit of an old case but I think it's still relevant for these things (BUT I am NOT a lawyer, so you may want to double-check)
In Alabaster v Woolwich plc and Secretary of State for Social Security [2004] IRLR 486, the ECJ held that UK statutory maternity pay (SMP) legislation does not comply with EU law in that a woman who receives a pay increase before the start of her maternity leave must be entitled to have the increase taken into consideration in the calculation of the earnings-related element of her statutory maternity pay, even though the pay rise was not backdated to the relevant reference period for calculating her entitlement under the Statutory Maternity Pay (General) Regulations. Under the current rules, a pay increase during maternity leave is only taken into account in the SMP calculation if it is backdated. The ECJ reasons that the principle of non- discrimination requires that a woman who still has a contract of employment or an employment relationship during maternity leave must, like any other worker, benefit from any pay rise which is awarded between the beginning of the reference period and the end of maternity leave. ?To deny such an increase to a woman on maternity leave would discriminate against her since, had she not been pregnant, she would have received the pay rise.? Since the decision was based on the equal pay provisions of Article 141 of the Treaty, it is directly enforceable against all employers, regardless of when the Government changes the SMP rules. This means that if a pay increase takes effect between the start of the SMP reference period and the end of the period during which SMP is received, the woman should be awarded the pay increase with effect from the same date as other staff. Interesting issues will now arise as to the rights of women who were incorrectly paid SMP by their employers and the time limits on any retrospective claims.