or you could get a volvo s60 d5 like I have although I paid more like £4k for it, can prob get one for £3k that a bit older or not quite as specced up. It'll do 45-50mpg normal driving, getting on for 60mpg on a decent motorway run at 70mph.
passat/audi a4 130PD will do a bit more MPG but at the expense of a bit of performance