I've been considering a small diesel as well - the Renault Clio 1.5 dci 65 seems quite attractive as it's posted at 65mpg and can be had used for not a lot of money at all. In its case,
http://www.spritmonitor.de lists anything from 43mpg to 75, with an average of 58mpg, so I think that 60mpg would be a reasonable achievement in the real world without hypermiling or modifications.
I've only ever driven petrols up until now and so I have no idea if the extra mpg will be worth suffering the foibles of diesel engines. The 1.2 petrol (closest power output) is posted at 48mpg, making the diesel 35% more economical.