Why they messing with my pay??

I woke up this morning to find two envelopes in the mail. Both for me.

One was from The Guardian with student vouchers inside, which made me happy as I don't like paying full cover price anyway.

The second was from the council [who I work for].

It read along the lines that there had been an 'administrative error' in the processing of my wages meaning I've been receiving enhancements on top of regular pay.
What it did not say was how long this has been going on for.

You all know about my money problems at the moment.

They want to see me at the end of the month to 'resolve' the problem.

I am seething. How long has this been going on for and are they going to want my money back? Because I'm reluctant to give them a penny. They should be held responsible for their own mistakes.

I'll be seeking professional and legal advice about it because I'm not happy to have been let down this way at all.

Perhaps I'm over reacting a little, but I honestly feel I work hard for the money I get and I don't appreciate this at all.