I have a year 2000 Astra Club 1.6. Despite having my car serviced, the mechanic hadn't switched off the 'service due' indicator. I had to phone the dealer and ask them how to do it. It's a while since I did it but I think it was...
step 1: with ignition off, hold down the button that zeros your trip meter.
step 2: put the key in the ignition and turn on the ignition (not the engine) while holding down said button.
step 3: the message should have dissapeared and the trip meter show zero. Release the button.
It's worth a try so hope this helps