Press and hold the trip reset button when the car is started for around 10 seconds. A menu will appear. Check your VIN number beforehand and add up the last 5 digits of the Vin. I.e if your VIN ends with 32454, then you need to remember the number 3+2+4+5+4=16.
Now hold the trip reset button for around 2 seconds and you should see the menu change - now press the trip reset button a few times until you reach no. 19. Leave the button and it will say "lock" or something to that effect. It will then ask you for a code, in this case 16. press the trip reset button until you reach 16, and leave the button.
You have now umlocked your hidden OBC menu. Now you need to press and hold the button for 2 seconds to exit a menu item, and then press it again briefly to enter the next menu. On my car the water temp is under menu no. 7.
Unfortunately you have to do this every time.