From my reading of this rather confusing situation the OP isn't owed anything.
The partner bought OP a car because OP couldn't afford it herself. The car will be worth less now than when they bought it. The OP has to deduct the depreciation from what she's owed.
The example that's previously been quoted, which as far as I can tell is just plucked out of thin air and not from the OP's posts, is
Car cost £10000
Partner paid £10000
OP has repaid £2500
Car can be sold for £8000
Both OP and partner lose 20% of what they've paid, so partner gets £6000 of their net £7500 and OP gets £2000.
That example is wrong because the partner shouldn't be losing money on this. The OP needs to deduct the depreciation from their own investment, so partner should get £7500 and OP is left with only £500.
If the depreciation is £2500, the OP should get nothing.
If the depreciation is over £2500, the OP will need to continue making payments until the full amount is paid off.
It's really just a typical leasing scam scheme, you pay the money to drive the vehicle, but you will never own it. Only difference is the finance is provided by the OP's partner, not an independent company.