How can someone take finance on my car when I own it?

[jemu]

I have a problem. I bought a car in 2005 - cashplus the part ex of my old car. The garage did a HPI check and it came up clear. I had troubel with the car from day one and after 5 months wet to go and part ex the car at a Fiat garage. They were willing to take this car in part ex for a stilo. However, they did mention that another HPI check had recently been done on it cos he asked me if I had been anywhere else to part ex it. I didn't think too much of it. A few days later I decided I did not want the Stilo and car finance hanging over my head and kept my car. A few weeks ago I went to sell the car to a garage to buy my partner a van for work and they told me the car is now showing as being on finance. I have spoken to then finance company and they won't take it off finance even tho I own the car. I can't find my receipt for proof of purchase and they won't accept a copy of my mum's bank statement showing the money being withdrawn. What can I do?