Looking at this but unsure of its value, autotrader guide says its £13250 as a private sale
https://www.autotrader.co.uk/classified/advert/201710110189844?sort=price-desc&postcode=b975qe&make=BMW&model=Z4&radius=200&transmission=Automatic&price-to=25000&minimum-badge-engine-size=3.0&onesearchad=Used&price-from=10000&advertising-location=at_cars&page=3
https://www.autotrader.co.uk/classified/advert/201710110189844?sort=price-desc&postcode=b975qe&make=BMW&model=Z4&radius=200&transmission=Automatic&price-to=25000&minimum-badge-engine-size=3.0&onesearchad=Used&price-from=10000&advertising-location=at_cars&page=3