Good question - I hear diesels aren't great on MPG when cold. In the case of the motorway trip, the car was fully warm (temp needle at 12 o-clock) when I hit the motorway and re-set the MPG.
My 'tests' in urban driving have included my very short commute of just 4 miles for the last few days. I reset the MPG indicator about 35 miles ago and the average still moves quite noticeably if I cane the car. After the motorway run (only about 20 miles), the average came down quite quickly from 60mpg, but settled at 35-36. When I set out from cold in the morning, I see the average drop by a few tenths on the gauge in the first couple of miles, but then it starts to climb back to circa 36mpg. So it's very unscientific and I'm driving quite gently, but it's clear that the average is still moving and I figure that if it's rising a little from 35 mpg, then I must be making 35+ mpg on a given trip.
I suppose I should bite the bullet and re-set on a cold start commute. I'll try that tomorrow. Have a horrid feeling it won't look good at all ...
Overall, I'm impressed with the mix.