1. Is this a hire purchase agreement? Everyone is assuming it is, but you only say it is car finance, which could be simply a credit agreement where you own the car but have borrowed money in order to buy it.
2. If it is HP then the finance co. own the car & can repossess it - subject to Court approval if necessary under the legislation.
3. If it is not HP, then get the car from her & sell it to pay off as much as you can off the debt.
4. You could certainly take her to the small claims court. You had a verbal contract with her that she would pay for the car. She has failed to do so. Your problem will be that she will deny there ever was such a verbal contract, & the Court will have to decide on the balance of probabilities whether you or she is telling the truth. Also, has she got any money? There's not much point going to Court, getting a judgement in your favour & then finding you still don't get anything.