I work in a department where I was the only supervisor. However I went on maternity leave and therefor someone needed to fill in for me whilst I was away. However I have come back and she is still employed and working in my department. I found out yesterday that her salary is somewhat more than mine, apparently being in a different department, even though the roles are the same can mean a difference in salary, this was before the new act of equal pay for equal work came in. So I need to know can I claim discrimination. I did ask our HR rep as to why I can't claim to match her salary, and she said that because her salary is based on what she was offered in a previous department they can't decrease her salary now, but it doesn't mean they need to increase mine either. 

She is coloured and I am white and we are literally doing the exact same job, she doesn't have more experience than me, she doesn't have additional qualifications that can justify the pay difference, they are literally just saying its that way because that's what her salary was from being in another department.

Thanks in advance