about how I was apparently overpaid tax credits from years ago
I KNEW it would be a supposed tax credit overpayment - this bullshit of theirs has been an issue for many people.
Seriously contact your local welfare rights office TODAY before you speak to dwp again.
They will keep you right and can even speak to them on your behalf. I know you are probably thinking you can represent yourself but honestly the rules are SO complex and the dwp so unreliable I would strongly advise against this.
A welfare rights rep will know the rules and if they're trying to fob off or even outright lie about things and will have the information, knowledge and experience to challenge them.
It's really quite illuminating to be on a 3 way call with dwp, yourself and an advisor and hear the dwp trying it on and the advisor say something like "isn't that against x rule/statute?" And the dwp person fluster and flap because they've been caught out!
Not because the dwp person doesn't have the knowledge but because they do and will try and deny claimants their rights.
I'll probably get flamed by somebody for saying that but it's honestly been my experience. I even have 2 people in my own personal circle who work for dwp and when I've discussed such incidents with them they're aware that some staff try this nonsense on, some offices are less...morally scrupulous than others. It's an open secret within the dept though denied publicly.
You need to arm yourself with info and support.