For a 1.6D to only give 34mpg something is either seriously wrong or you drove it everywhere in 2nd! I'm getting around 60-65 at the moment from mine (59 plate, 16k miles) and last summer I was able to get mid 70s on the same 30 mile commute. Take the car for another test drive, reset the trip computer and see what happens. You'd need a half decent distance (20-30 miles?) to get an idea and even this won't be 100% accurate, for that you'd need to log the exact mileage and fuel put in for a tankful or two.