unequal pay - discriminaton and how to proceed

I am female and found out that my two male colleagues who do exactly the same job as me are being paid £4000-£6000 more than me.

I found this out through a friend from at work who can access to the HR database and who told me.

I am going to raise this with my line manager today but I don’t know how to approach the situation given how I found out the information.