As it's a new to you car, I wouldn't assume the gauge is anything like accurate.
The ONLY way to establish with decent accuracy is to drive from brim to brim on a tank and work out.
It certainly should be better than this, the last tdci or whatever it's called focus(09)I drove, I was getting about 38mpg, driven gently but only had around 1500 miles on it, and a heavy footed driver!!
tabetha