hi, i am just a lay person not a finance bod, but here is my take on things.
I dont understand on what basis you think you don't owe this money. you say you have now paid 50% and so it should be written off - but why? When people take out loans, they agree to pay 100% back, not 50%! When you have car finance, when you ahve paid a certain amount of the payments you can just hand the car back and exit the agreement, but this dosen't apply here as you do not have the car to hand back.
Paying PPI cover does also not get you out of paying, as i presume the cover was along the lines of it would cover your payments if you were ill or redndant, not if the car was a write off?
as an analogy, imagine you had taken out the loan to buy something else, say, a telly, which then got stolen. The loan company arent going to be a jot interested in the fate of the telly, jut about getting their loan back, and you would still be liable to pay it until it was all apid back. Apply this to your situation now, the loan cmpany arent a bit interested in the fate of the car, just that they lent you money and you are not paying it back to them. you had the option of GAP insurance and fully comp insurance, but declined to tAke them, that is not the loan company's fault