I used to do a 100mile round trip to work, but it was all country lanes with NO dual carriageways.
Best time was 55minutes and worst was over 2 hours.
Now I do a 40 mile round trip, but because it's toward the city it means rush hour traffic, takes about an hour which is rediculous.
If you want a CHEAP diesel, consider the Clio. All the Renault cheapness, plus "might-as-well-be-free" tax and 50mpg with average driving... Did a 250mile motorway journey yesterday though and flat out (between 110 and 120 GPS) does 25mpg.
Got caught in 2 sets of traffic though, both crashes, one of which involved a Ferrari 355 and a TVR Tuscan!! And... a fiesta, corsa and zafira..