Oooh! I've had dealings with Concentrix. It's an American company and has been tasked with chasing overpaid tax credits. Anybody who has ever had any dealings with HMRC about tax credits knows that the system is barmy.
A few years ago, I was overpaid by about £3000. I knew I was receiving too much money, so contacted HMRC and tried to get the payments stopped, but was told to wait until the end of the tax year. I waited and eventually my tax credits were stopped and asked to repay the overpayment, which I did.
After about another six months, I received a letter from Concentrix, asking for another £2500. I wrote back and said that I didn't have £2500 (which was true) and asked how they had worked it out. I received back a letter that I could pay in instalments, but still no explanation. I wrote back saying I was happy to pay, but I didn't know why I owed the money. I then received a letter with a date for a court hearing. I rang Concentrix and got an American style sales brush-off, so I contacted HMRC direct and explained the situation. They told me I didn't owe any money, but I still had to attend the court hearing.
The next thing I knew I received a demand for court expenses and yet another demand from Concentrix, so I contacted HMRC directly - yet again. The woman on the end of the phone couldn't have been more apologetic and said it was happening to a lot of people. Eventually (after about 18 months) the whole matter was dropped.