Capquest - advice needed

Dulog

I'm new to this site, so I hope I'm posting in the right place!

I recently paid off a credit card debt taken by Capquest. I paid by standing order. The end of last year I had to contact them after noticing there was (and is) a payment scheduled 18/10/19.

The agent told me he knew nothing about this and that the debt was paid off and the account closed, but also noticed that they owed me £37.50. A few days later they phoned to confirm this and sent a cheque.
This week they started phoning me again. I called back and was told first that I owed them £34 - that the account had gone to 0.00 but jumped up to £34 "for some reason". The agent did some checking while I listened to music, and came back telling me that either they owed me £3.70 or I owed them that amount (not the exact amount but something similar). They will phone back when they know.

What worries me is that a "closed" account can suddenly reappear with me perhaps owing them again. Also, all this is over the phone. Any advice would be aporeciated, sorry for the long oost!