Doing a bit of looking about as you do and spotted these two, or should I say one as it's got to be the same car. I know one will come with a good warranty but that's about it really.
http://www.autotrader.co.uk/classified/advert/201501270464451/sort/default/page/2/postcode/dl11eq/make/bmw/body-type/convertible/keywords/3.0%20si/model/z4/usedcars/onesearchad/used%2Cnearlynew%2Cnew/radius/1500?logcode=p
http://www.autotrader.co.uk/classified/advert/201501270464451/sort/default/page/2/postcode/dl11eq/make/bmw/body-type/convertible/keywords/3.0%20si/model/z4/usedcars/onesearchad/used%2Cnearlynew%2Cnew/radius/1500?logcode=p
http://www.autotrader.co.uk/classified/advert/201501270464451/sort/default/page/2/postcode/dl11eq/make/bmw/body-type/convertible/keywords/3.0%20si/model/z4/usedcars/onesearchad/used%2Cnearlynew%2Cnew/radius/1500?logcode=p
http://www.autotrader.co.uk/classified/advert/201501270464451/sort/default/page/2/postcode/dl11eq/make/bmw/body-type/convertible/keywords/3.0%20si/model/z4/usedcars/onesearchad/used%2Cnearlynew%2Cnew/radius/1500?logcode=p