SFE demanding money I don't owe? What can I do at this point?

When I changed course last year at uni, the university mistakenly put me down as withdrawing, so SFE demanded £921 in maintenance grant repayment. The uni then corrected the mistake, and SFE said I no longer owe the money.

Then I kept getting letters, each more demanding than the last. I kept ringing, and they kept saying "you don't owe the money, it's just an automatic process sending out the letters". I was eventually told the cancellation process was having to catch up with the initial money demanding process, so they put a hold on my mail to give it a chance to do that. That was fine for a while, then I got another letter threatening court action if I didn't pay up. I was told the hold on the mail was specifically lifted by someone for an unknown reason (also they apparently can't do this without my authorization), so they put a hold on it again.

It's now a YEAR later, and they asked my permission to lit the hold on the mail so they could send me necessary correspondence regarding my current application. They said the £921 debt thing was totally cancelled now, the cancellation caught up with it and I'd no longer hear anything more about it.

Today, I got a letter saying that since I haven't paid up, they've got in touch with a third party debt collection agency that will be in touch regarding the matter.

I'm at my wits end; I've been told multiple times I don't owe this money and it's just an automatic process, but it's been a year and when does the process end? It's not just letters, it's debt collection agencies and threats of legal action, and I've been told I'd receive an email saying I don't owe this money but I haven't, so I only have the word of the people on the phone to go by. They've failed to provide me with physical evidence of the debt being cancelled, they're making all these threats and taking action, what can I possibly do that I haven't done already to make this stop? Also will it have affected my credit rating?