If you're buzzing along the motorway then at that point of the journey it is possible to hit between the 38-45mpg while driving fairly easily (well it has been for me). This is if you're driving at around 60-65mph and using the pedal just enough to keep you at this speed and no more.
Like others have said though, if you're then turning off the motorway and then completing the last few miles of your journey on small roads with many turnings then you'll be hitting around the 20-25mpg for these parts at best, which overall will bring down that average to the 30-35mpg for the journey total.
The trip computer is a blessing though. I didn't know it was even there until a few weeks into owning a 147.
Knowing you've only got a quarter of a tank left and being able to change the display to constantly read out the average mpg you're using at that time is great...there are moments though that this car is tremendously enjoyable when putting your foot down, and I wouldn't advice on missing out on that feeling just to increase your average mpg.
I previously owned a 1.4 Renault Clio, and I'd say my 147 uses about 20-25% more fuel overall for the miles being driven. It's completely worth it though.