JacqueslePeacock - my advice is sit down and do a hard objective and evidence based analysis of your qualifications, skills, experience and performance of yourself and each of your peers on your grade.
Write it down and tabulate it and then send the evidence to your boss and ask for your current pay to be justified against the criteria you have set out and against your peers. The organisation should respond seriously to your request. If they dont they are opening themselves to a claim.
I base this advice on the experience of my DW in recently applying for a public sector job. She was offered the job but at the very bottom rung on the pay scale for her role and grade.
She wanted the job but set about negotiating all the way to the top of the pay grade from where she was initially assessed. She did this by doing a very detailed objective analysis of the output of each of her male colleagues on that grade above her on the pay scale.
She presented the analysis in writing and and asked her prospective boss to justify why her male colelagues were on a higher level in the pay scale. She set out the objective crieria that should be used to decide her pay scale level. She clearly outclassed or matched each of the men on objective criteria including the highest paid one.
It turned out that despite putting in a very detailed CV and is no shrinking violet in negotiation that all of her interviewers (all male) had simply assumed immediately and with no justification that she was very very junior in her previous job and coming back to work after having had children she would just accept what she was offered.
Her boss eventually admitted they had wildly but not deliberately under assessed her experience and qualifications and agreed to correct the error. He admitted that they had also assumed she had exagerated her seniority on her CV like everyone does.
This example is a real life one and shows how unconcious prejudice causes women to be paid less than men who have the same or lower performance and qualifications.