When you say you've changed the battery, I presume you mean the battery in the key. "The battery change must be performed within 3 minutes, otherwise the remote control will have to be reprogrammed."
"In the event of malfunctions, programme remote control:
Switch on ignition; system will then remain in programming mode for 30 seconds.
Briefly press any button on the remote control unit while the unit is in the ignition.
The central locking system locks and unlocks to show that the remote control has been reprogrammed."
The quotes are verbatim from my Vectra manual. Might save you some pennies, but if it doesn't work I can only second the suggestion of going to the dealer.