Agreed.
I can easily get more than 400 miles during a motorway cruise, in fact I got nearer five on the way back from the Ring last year. (seriously effective aeroynamics on the Zed)
If I have a 'poor mpg month' and spend lots of time queueing, and driving around towns and cities, I usually get sub 350. In the summer (and hence usually more enthusiastic, a bit less!)
I find the Zed to be hugely economical for what it is, however, you need to be driving to it's eco strengths (i.e. the motorway) to realise the economy.
I guess that's why I am asking - I never queue, I live in the middle of nowhere. Even if I'm stuck behind someone, I'm doing 40. There are maybe 5 junctions between home and work (15 mile drive) where I have to come to a complete stop, just before pulling out.
There's a big difference between a constant speed with no gearchanges on the motorway and your B-road commute where you've indicated that there are plenty of gear changes, plus you're covering the miles slower. That's why you won't get motorway MPG.