In short the answer is NO, but. . .
Is the house in joint names? if yes then apply for sole possestion, after 4 years you couldn't have payed that much off your mortage and i can't imagine your ex putting much of a defence to claim half the debt (unless the house has a lot of possitive equity)
Make sure all corrispondance is now done in writing!!!! No verbal under any circumstances!!!!! Write to her mentioning the above making it clear that she has previously refused offers to pay half the mortage and thus should have no claim to the house.
The registered owner of the car is irrelivant, you WILL have to pay ALL the loan as it is joint - If you want to keep your credit rating then pay the loan and as long as it is below �10000 (�5000) claim, take her to a small claims court. You Will get the judgement and if she refuses to pay the baillifs will take possestion of the car.
Court can seem a daunting process - it;s not, but it can take a long time and your bank balance will suffer the brunt until a judgement is passed so START NOW.
The form you need is a N1 form, she may even back down when she recieves the court papers. But REMEMBER no verbal communication form now on!!!!!!! You need to substantiate the veracity of your claim(s) and send everything recorded signed for.
Good Luck and start researching (unless you want costly solicitors fee's)