Disco 300Tdi Auto EDC will give you 26mpg overall.
Had several over several thousand miles, and you can get more or less, but that will be a fair average.
Most are EDC with electronic fuel control, it does cause problems, but is the one to go for, as it makes the auto quicker than the manual - not that you'd notice
(However, should you be going overland, then the mechanical pump is preferred, as more reliable, but very small production window around 94-95, and some very late 200Tdi's)