Thanks for all the great replies, especially kikkthecat for that detailed writeup! I think I'll keep an eye out for a 2.2CDI, but I'm after the X-Long so might be in for a bit of a wait for the right car.
The extra long doesn't look too bad with roof rails (standard on the Viano, I think) to make it look a bit taller. Without them it looks like a train
Mobile phone pic. of a V6 extra long Viano I regularly see around Charing X: