Mushroom25's link deals with the Supreme Court's decision as to whether the women wanting to bring an equal pay claim (which would normally have been time-barred) still had the right to do so. It doesn't though cover the actual issue at the heart of the problem, which is one of 'equivalent work'.
As an analogy, let's say that a supermarket chain pays its checkout operators (both male and female) £12 per hour but pays its warehouse staff (both male and female) £14 per hour. On the face of it, they're within the law as any checkout operator gets the same pay as their colleague on the next till (irrespective of their genders) and similarly all warehouse staff receive the same pay, irrespective of their genders.
However their checkout operators are mainly female, whereas their warehouse staff are nearly all male. If the women on the checkouts can convince a court that their work is actually equivalent to working in the warehouse, they'll have a valid claim for discrimination on the grounds of gender, thus forcing the supermarket to pay them the higher rate of pay (and to give the checkout operators back pay, to make up the difference, as well)
So it was in Birmingham, where staff such as teaching assistants (who're prominently female) managed to convince a court that there were other jobs within the council, with predominantly male staff, which were 'equivalent' to their own but where those male staff were being paid at a higher rate. The court therefore ruled that Birmingham City Council would have to fork out a massive amount of back pay to the women workers, which they simply hadn't got.
Such 'equivalence' claims are becoming far more common, with many employers (both in the public and private sectors), who thought that they'd been were fully with equal pay legislation, suddenly finding out that they'd not and getting left with massive debts as a result.