Is it the spanner light you wish to put off?
Switch off ignition.
Press and hold in trip meter button.
Switch on ignition.
Wait until countdown reaches zero then release trip button.
will that restart to mile age countdown again till next service '20,000 mile' aswell, would anything happen to the car if you did this and the spanner light had not come up yet
This will reset the counter to the next service.
If you want to be specific about when that will be then you need a suitable scan tool to reprogramme the service interval.
Not sure if it will reset if the spanner light isn't on as I've never actually tried it.