Have a look at your speedo and you will see a little black stalk sticking out at the bottom towards the right.
Make sure you can see the 'service required' or 'change oil soon' warning message by pressing the selector button on the end of the wiper stalk.
Press and hold the black stalk on the speedo. It might take a bit of pressure to depress it (mine did) but if you keep holding you will first see the warning message begin to flash then (keep holding) it will reset getting rid of the 'change oil soon' message and flashing spanner and also resetting the interval in the 'service required in...' message.
geordiemark
July 2008