On a recent run to scotland and back, around 1000 miles, I averaged 47.5 mpg.
Had nearly 500 out of a tank once but if I want to enjoy the car then it drops dramatically to the low 30's and around 320 to a tank.
I imagine if you drive a diesel economically you could reach neaer 60mpg.
Haven't you already got a ZS TD Gazza?
Average out of my 120 is 35-40mpg on a long motorway speeds run. 30mpg if you're lucky around town. Lowest I've run the tank was 370 miles. The light was on and I was bricking it looking for a garage. Mind you, I started off with the tank brimming.