That particular engine should return 33+ mpg driven well.. That will deminish in town runs or on fast motorway journeys.
Don't take too much notice of the "140" statistic you get on the computer, it takes into account all the averages of mph and mpg etc... So you can put 20 quid in one time and it will say 190, and other times it will say 140.
I got a 330 ci Msport 2003, have had it a month and I consistently get 137 miles, all local driving and im always putting my foot down cos its still honeymoon for me, all from £30 petrol from reserve light to reserve light.
so I think 140 from £20 is very good! :jaw-dropping