I to am sceptical John but can get in excess of the quoted figures but I do drive like a grandad - well I am, I rarely exceed 60 mph or 2000 rpm, I only use my brakes when absolutely essential and will let the car slow down going up a hill rather than increase the throttle opening - I hear a chorus of 'boring' but I am more interested in comfort than speed now,, hence the Citroens.
The 2.1td XM estates that I ran for over 15 years would regularly return in excess of 50mpg on our annual north of Scotland trips but that 2.1 is probably the best IDI engine ever made, very smooth and better consumption than the 1.9. The official figure for 2.1 was I believe 38mpg, I don't think I ever got mine below 40mpg
A couple of years ago I averaged 67.8 over 680 miles in the 406 Hdi estate and I have the read out to prove it, well over 1000 miles on a tankful.
There probably are not many people that drive like me but it shows what can be attained by careful driving, diesels will reward you for this more than petrol engines.
I developed this driving style when I was working and covering around 30k miles per year, I found it so much more relaxing.
Peter