34.7 to 34 is a fraction of a change. Going up one hill can make your average mpg on an entire tank (600 miles) drop more than that.

Seeing as you have only had the car a week and a half I would wait until you have driven it a bit more to get more of an idea of what average mpg it will do.

With steady driving though, you should see more than 34 on a diesel... Try driving a bit smoother - accellerate slower and slow down into corners/roundabouts more in advance using engine braking rather than the brake pedal.

On my Laguna diesel I see anywhere from 48mpg to 42mpg average on a tank. It all depends on how you drive, what roads you drive and probably the weather as well