I have the same problem myself. I have been told there is no adjustment and it is done automaticaly. However, I have spoken to Vauxhall who say they will adjust it for £45 but wont cough to telling me how (jobsworth's). I recon there is adjustment under the car by removing the center exhaust box, it is situated above this. I have with great difficulty had the center console out but no visable means of adjustment there.
The reason I am not convinced there is no adjustment & it is not done automaticaly is when in the past I have had it serviced by Vauxhall, the handbrake has less travel than before it went in. Now I have fitted new original rear pads my handbrake has much travel.
Alan, December 2006