Vito 109CDi Fuel Consumption query

I really need some feedback from other new style Vito van users about fuel consumption.
We purchased 2 new style diesel Vito 109CDi Compact vans at the start
of this year.
They were some of the first of the new range in our area. However, we've found the fuel consumption to be very poor indeed.
Typically, we seem to be averaging between 25 and 28 mpg.
Our vehicles are mainly empty, rarely carrying any weight at all. Most of the driving is in urban areas, yet our previous VW Transporters managed much better fuel consumption.
I recently was able to borrow a demo 109CDi, and monitored its fuel consumption over a 2 week period. It did much better, averaging around 36 mpg.
The 109CDi has a 2.1 litre engine. I'd have expected a modern diesel engine to give good economy, but so far that's not been our experience.
I'd heard a rumour that there was a known problem on some new style Vito's where the rear axle sufferred from excess friction, causing this poor fuel consumption phenomena, but have so far been unable to verify whether this is true or not.
Anyone else out there driving a new style Vito 109Cdi get any better results than us?
Thanks in advance for your help
Best regards
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