This how it was done on my old vectra
1.With ignition OFF,press and hold reset button.(trip mileage,although I believe time clock on some).
2.With button still held in,switch ignition ON with foot on brake pedal while continuing to hold down button.
3.After 2 seconds,InsP should flash then change to ----.
4.Release button and display reverts to trip miles.(or time?)
It should work as I've just had to do the same to my vectra (56 plate) as the garage forgot to reset it and I couldn't be bothered to take it back for them to do.
Mine is an auto 3.0 diesel.