Forced to pay monthly installments for a written off vehicle.

I took a car with planet42 and 2 weeks later the car had an engine failure.send it for repairs and the quotation was 83k.they told me its uneconomical to repair they writing it off so i asked to keep the car and repair it myself.they agreed,they sent me a paid up letter and informed me that i dont owe anything towards the car,my account is settled.2 months later they send me an email saying my account is in arrears.i asked how because they closed my account and cancelled instalments.i disputed the arrears but they wanted to force me to continue paying instalments for a written off car which they said its paid up.i lodged a complaint with ombudsman and now they are calling me saying my contract has been terminated and i must return the vehicle or they will send repo agents.i dont understand how because the case has not been finalized with ombudsman yet they are busy harrassing me.