There seems to be some confusing figures in this topic.
New car - 23k 'cost', then pay 11,192 (monthlies + deposit) over 3 years. If you trade in/hand back after 3 years you have paid 11,192. If you sold it privately, I think it would probably be worth 17k, pay off the GFV of 13,800 and you are left with 3,200. Car has cost you £7992 over 3 years.
Used car - pay 15k now, worth 8k in 3 years, which is 7k. If you put some of that on finance there will be a cost for the interest. Also factor in warranty/additional maintenance against a brand new car.
To me, the current deals make buying a new one a no brainer.