Is your finance via bank personal loan or an hp finance deal specific to the car? If against the car you need to clear the full amount else the new buyer will be liable and you will end up in hot water.
If you go for something ridiculously cheap to insure, maintain and run such that your monthly outgoings reduce by a significant amount then you could justify selling your ed30 below your settlement figure. Say a citroen c1 (sorry this painful) ... And running that "car" saved you say £200 per month then in one year you will have clawed back £2400 say.
The trap of negative equity eh? I bought vw cos I of retention values, of course all that has went tits up the world over, may just have to ride it out myself and keep the gti until the bitter end. Or downsize now and accept less for mine but also safe in the knowledge that I am paying less for it's cheaper and duller replacement. Perhaps an economist can e plain the dynamics of this equation?