Good luck OP. I also work in the public sector. I got offered to take over the role of a man who had just left, but several bands (not even points, whole bands!) lower on the pay scale for the same job! The excuses were that they weren't sure of the job description, it might change, they were doing me a favour by giving me experience of a higher level of work (erm thanks, but I'd like to be paid for it!), they also said it was because I had less qualifications, experience etc until I pointed out that I had the same level of experience and slightly higher qualifications. At one point they said the man had 'additional duties' but wouldn't tell me what they were, as this was irrelevant (even though this was the whle bloody point, whether or not we were doing the same job). It turned out these were duties I was doing too. I negotiated, challenged etc and couldn't get anywhere informally, I had to raise a grievance, which I lost, and only won on appeal.
I may not even have made the grievance had I not been still pissed off about being refused the job below that one the previous year because it was 'a man's job', so they gave it to a man with no relevant experience or qualifications instead of me with lots of both. I didn't challenge it at the time because I thought I would just work harder and show them how great I'd be at the job, so that they would give it to me when it next came up. Which they did, but without the pay.
It's all sorted now, but the moral of the story is there are definitely bastards in the public sector as well. It's not as transparent and pro-equality as everyone seems to think it is.