You'd probably expect to see a car in this condition up for more than £20k at a dealer and not far off that if sold privately. If you put the hours in selling it (decent prep, decent photos, good description) and get it in as many outlets as possible (as well as dealing with all the phone calls) you'll max out your money ... as opposed to placing it with a dealer or an auction (less hassle but they take a cut). Make sure you hit the BMW specialist titles and websites as well as more mainstream.