For the first time, the UK's biggest employers are having to reveal the average wages they pay men and women. At the same time, the BBC and many other organisations find themselves in battles over equal pay. Almost 50 years after the passing of the Equal Pay Act, why are women still not being paid as much as men? Jane Corbin travels the UK to meet the workers, from supermarket staff and council carers to BBC presenters, who are fighting for equality - even if the costs run into the billions.