I’d phone them up and ask them what it’s all about – only way to know for sure.
Then ask them to confirm it all in writing. I’ve been down this loop with the tax man – was told everything was fine and the money was mine, only to be told 10 months later that it was wrong and I needed to refund the money, which I’d spent. The call I had placed had absolutely no bearing at all, even though I’d recorded times, names, etc.
It was a reasonably significant amount of money too, which caused me no end of grief. There’s no option to repaying the money, they’ll take great delight in sending the heavy boys in to take your TV away (to paraphrase one unhelpful HMRC employee).
Pay it in, wait for a coupla months; if you don’t hear anything, fillyer boots!
Change coupla months for a coupla years.
If it’s a significant amount of money and you’ve got the willpower then stick it in a savings account and leave it for a few years.