Hi
You can't "bypass" the immobiliser!
However, any Xpart Autoservice Centre (http://www.xpart.com/html_content/specialist.htm) has access to the former Rover database containing all the secret codes relating to each car that was made.
AFAIK, each car has three codes: key-cut, immobiliser and radio code. By presenting your proof of ownership (i.e. V5c registration certificate and Cert of Insurance) a local Xpart centre will be able to provide you with these for £10-£20 fee.
So long as you're willing to live without the security of the immobiliser, you don't need to pay for any work to be done. All you do, is disable the immobiliser by entering the code via the driver's door lock. The immobiliser will remain inactive indefinitely - even if you lock the car with the key - until it is locked again with its own radio-fob.
SS, November 2007