Let's look at the facts here:-
1) You are the second owner I believe?
2) Car was previously used as a dealers demo car? If I remember correctly from your previous posted.
3) The car is less than a year old and has covered 15k miles.
4) It's been involved in an accident, but has been repaired to a good standard.
5) The car is in good condition and well looked after.
6) I'm assuming it's had it's first service and a new set of tyres?
7) The car is a decent spec in terms of options extras and a desirable colour.
The bodywork is in good condition?
9) You have outstanding finance which is highly likely more than the car is worth? Remember the interest gets added on at the beginning, so paying back early means you are paying for the full term of interest still.
10) There have been mixed issues with the car, repaired under warranty.
Based on the above, I honestly think you'll struggle to get over £20-22k for this. Based on current 'used' prices for lower mileage and ones that haven't been involved in an accident (which always puts people off). You are still fairly close to 'new' prices, particularly if someone can secure a decent fleet discount. People would also prefer the newer plate.
Most would rather pay a little extra for one that hasn't been damaged (repaired) and hasn't been used as a demo car.
You mentioned that you better half is looking to buy an S1, have you thought about trading the GTD in for it?
You mentioned also that you got offered an allowance instead? Surely if you took that + claimed mileage back you'd at least be breaking even?
From memory you used to be a car dealer yourself (again from another post), you are probably better placed to answer you own question than any of us are! :-)