I've now got my trip-computer totally accurate, and in a moment of boredom, i typed this out. I hope it helps and makes sense. Here goes . . . . .

Before you start, do the following sum – Computer MPG divided by Actual MPG, and then multiply the answer by 1000. (Before the mathematicians moan, the trip works in L/km – inverse to MPG!!!) You should get an answer around 1000 (say 850 to 1150 ish)

1) Put the key fob in the dash, get the car to the point where just the oil light is on (not the ABS, airbag etc) You may need to press the start button a few times to get here, but do it without your foot on the clutch.

2) Press the trip reset button (small thin button on left of instrument cluster) for about 15 seconds, during this time, a yellow triangle will appear – keep holding the button down until the display shows something similar to YZ12789. Above this in smaller text there will be “01.00”

3) This is important. Add up the five single numbers in the YZ bit – eg 1 + 2 + 7 + 8 + 9 = 27. This (27) is the unlock code.

4) The 01.00 bit is a bit like a mobile phone menu. You are in menu section 01, looking at sub-menu 00

5) Now hold the trip reset button for about 5 seconds until 01.00 changes to 01.__. When this appears let go of the button.

6) Now press the trip reset button repeatedly until you reach 19.__ . Below it you’ll see “LOCK” and below that “00”. At this point stop pressing the button and pause a moment.

7) Now press trip reset button repeatedly until the “00” reads as your unlock code (e.g. 27), once you reach this number, pause for a second and LOCK will change to “UNLOCK”. The hidden menus are now unlocked – there are 21 different menus and a few sub menus in each. I’d advise you to leave them all alone except menu 20.

8) Press and hold the trip reset button for a few seconds until 19.__ is shown, then release.

10) Press trip reset once so that 20.__ is displayed (menu 20 is the MPG menu) Underneath that will be the number “1000”. This is the multiplication factor for your mpg.

11) Now remember the number I got you to calculate at the start (around 1000) you need to change the factor to this number. It’s a bit like setting a cheap digital watch and although there are 4 digits ( 1 0 0 0 ) they are split into 3 parts (10, 0, 0)so you have only 3 numbers to change. For reference, my number was 888 – but you could have 945, 1034, 1155, 1253 etc etc

12) Press the trip reset button once and the ‘0’ on the right will cycle from 0-9 , press the button when it reaches the number you want (8 in my case). The 2nd ‘0’ then cycles 0-9 – do the same again. Finally, the remaining 2 digits (10) cycle between 7 and 12 – press the button when you want it to stop (again, 8 in my case). So I ended up with 888 as my figure. This is the factor changed – well done!

13) Finally, you need to relock your menus. Press the button for a few seconds until you get 20__, then press repeatedly until you get to menu 19.00. At this point you either enter the unlock code again (the sum of the five numbers) or sometimes mine automatically locks if I pause for a while. Either way, make sure you see “LOCK” and then press your key fob to remove your key. Finished!

You may need to adjust that factor slightly, but over time you can fine-tune it to be perfect. Good luck men!