Barclays/Bms ltd debt

Hi all this is my first post so please bear with me.
A number of years ago (approx 2004) I had a business loan with barclays, business didn't take off and ended up with £6k debt, paid some but then it was referred to BMS LTD, offered to pay £5 month which was agreed, have paid this every moth since, been out of the country for a number of years, now Back only to notice that £5 is paid to them every month, but then a few days later its sent back into my account. I've tried calling them on 0845 241 7880 and sent e-mails to the lady I dealt with. no reply on email and then phone number is dead. Don't really know what to do about it all. But it can't help my credit rating. Any help or advice please