My dad invariably does about a thousand miles a week, almost all on the motorway, in his company car at the moment and in previous vehicles he's owned himself.
Currently, he has a Mitsubishi L200, which has to be the most fuel inefficient 2.5l TD ever, averaging about 25MPG at 80 on the motorway, I kid you not! He had a Passat Mk.6 2.0TDi PD for a month as a loan car, which had been run in properly and was returning very close to 60MPG! the best of anything so far. Prior to that he had a 1.9TDi Passat Mk.5.5, which did just over 50 to the gallon on the motorway, and before that a Laguna 1.9dCi which was ever so slightly better on fuel (I think it's taken them a while to get PD diesels to be similarly economical to the
Talking about his own cars, the best was the 306 HDI he had before he went back into working for a company, which returned about 55MPG despite being 1st generation
technology - I suspect this was due to it being quite a bit lighter than most of the cars he's had since. He had a Mondeo before that which seemed to do similarly to my current Laguna, although it might've been a bit better round town due to not needing to work it so hard - turbos have got a lot more midrange/bottom-end grunt so tend to do better on fuel in stop/start traffic...
It seems to depend very much on how the vehicle has been run in because I'm seeing massive variations in the same car, and driving style isn't such a factor at M-way speeds.