Thanks for all the replies.
It would seem my X5 is running at the typical average of 17-18mpg. So unlikely to improve.

I have recently acquired a 2004 3.0i X5 Sport, auto, 102k miles, part service history, but reasonably looked after by previous owner. Spark plugs, oil and air filters changed recently. The attraction in buying the car was the LPG system which should make the fuel costs somewhat less expensive than ...

This worked for me as well! (2004 X5 3.0i)
After I also spent many frustrating hours researching other solutions.
The correction is immediate.
Brilliant. Now I do not have to find my old school French/English dictionary!
Many thanks