It seems to vary wildly! I have a friend with a Td5 auto who regularly gets 35mpg. My manual used to get 30-32mpg before I started modding it. These days with 265s, roofrack and rooftent permanently mounted, I rarely see better than 26 unless on a long motorway run when I can creep up to about 28mpg if I take it steady.
I've heard of autos getting 22-35 and manuals getting the same range, all very confusing. I guess its down to driving style, the road type (city v motorway), and the routes covered (flat and straight v hilly and twisty).
IMHO If you get mid 20s you'll be doing about the realistic average.