I have an end '07 reg'd 2litre expert the 120bhp version which seems to be averaging around 28 to 29mpg. I've had it back to the garage a couple of times now due to this issue and a load of blue smoke coming out the exhaust upon startup, including a lack of power compared to my old model expert from an '03 year.
Apparently everything is fine on it (which I find amazing) - I miss not being able to do over 600 miles to a full tank, and am now only getting 450 which costs over £90 for the fill.
The van is going in for a software update next week due to a recall so we shall see what happens then.
I have a mate who has a 2litre Citroen van, but long wheelbase and he regularly gets 550 to 600 miles on a tank, and if he tries, he can get 670miles.... my best so far is 480miles after driving slowly everywhere for the tank!!
Would certainly be interested to see if it is a known problem or whether it's fine. It is a shame it's far less efficient than the last model expert which was a cracking van.