Just for reference, I traded in my higher spec, similar age & mileage Z4Coupe Auto in Oct for £10,500 :
http://www.sytner.co.uk/car-search/5254399-bmw-z4-3.0si-sport-2dr-auto/
The dealer tried over winter advertising at £15,250 and has dropped now to £14k. Presuming they have performed any necessary work (Insp 2, brake fluid change, new rear tyres, etc etc) I would say its worth £12k from the dealer because of the spec and warranty (with nothing needed doing) .
This car I would say its not far off maybe around £11k is more realistic.