HELP!! VT Nightmare ending up with default on credit file

[Sims1]

Please help I need some advice, having voluntary terminated my vechicle due to ill health. Although not fully having reached the half point, had a final balance of £1,900 still remaining on the finance. Can the finance company place a default on my credit file? I am now registered as disabled and was not able to afford to pay this amount in full and had notified them of this. They then sent a payment plan of what I can afford to pay each month, sent this back but had not heard nothing back. Can I fight the default? as the debt has now been paid. *