Ok - that could be a problem. You said £2000 of a £21,000 car.
It was then written off and you had £12,125 as a result and got a new car.
So in short you got £12,125 for a car which you had only paid £2,000 towards, because the insurance paid out. I realise the insurance was in your name but you had only paid 10.5% of that value.
Therefore had it not been for him paying the difference on that car you would not have had the payout of £12,125.
Using that same formula then this means you would be entitled to 10.5% of £12,125.
This is because that is the % you own.
Therefore you would be entitled to £1273.13
Your ex would be entitled to £10,851.87
You could reasonably split insurance costs etc., but sadly you only had a big insurance payout because of your ex's contribution.
Can I clarify anything for you about this today please?