ok, found this on the search.
Mine is so far out, I think I'll be adjusting it a few times to get it right
1) Key fob in
2) Press Start button without pressing clutch or brake if you have an auto box
3) Electrics / ignition is on
4) Press and hold trip reset button for about 10-12 seconds until OBC display changes showing something like YZ12345 where mileage normally displayed, at the top of the OBC display in small characters you will see 01.00 (Menu 01, sub-menu 00)
5) Add up the 5 digits, so here we have 1+2+3+4+5 = 15 which is your UNLOCK code, remember it.
6) Press and hold trip reset button for about 2 seconds until menu.sub-menu digits show 01.__, release button
7) Press trip button repeatedly until 19.__ is showing and you see LOCK and below it 00, release and wait a second or so.
8) Press trip reset button repeatedly until 00 counts up to your unlock code (e.g. 15), LOCK should change to UNLOCK. The extended service menus will remain unlocked even if you remove the key fob, until you come back to 19.00 to lock it again.
9) Press trip reset button for 2 seconds until 19.__ displayed
10) Press trip reset to menu 20.__
11) Below the digits 1000 should be displayed which is the current multiplication factor applied x 1000 (so it is really 1.000)
==== Notes for the next step
This figure can be adjusted between 700 and 1299 in others words a -30% to +29.9% adjustment.
I am fairly sure we are applying this multiplication factor to a l/km fuel consumption number, which is an inverse scale to mpg. I tried a factor of 810 and got 96.4 mpg on a short drive through a town.
For OBC reading an mpg figure 10% too high, I think the correction number needs to be 1100 to increase the underlying l/km figure which reduces the mpg result.
My OBC was reading an mpg figure 2% too low, so I think I need correction number 998.
The figure 1000 is adjusted as follows, the digits in the number are split 10 0 0
====
12) Press the trip reset button, the right most digit counts up 0-9 and goes back to 0 until you press trip reset button again. Wait for the right value and press trip reset.
13) Middle figure counts up 0-9, do the same again
14) Left most digit(s) count up between 7 and 12, wait for the right value and press trip reset.
I just did this, set the adjustment to 1200, then reset the MPG and drove about 20 miles. It looks a lot more accurate. I'll do the math with the next fill up to see if it is spot on, and adjust it further more if need be...