Hi
There is no single code for the immobiliser, it's individual to each car. There should be three cards (inside back cover of Owner's Handbook) upon which the first dealer should've written all the codes (key-cut, radio, immobiliser) for the vehicle.
If you have no record of the codes, you can request them (with proof of ownership i.e. V5c, Cert of Insurance etc) from an ex-Rover dealer. A lot of them are now Xpart Service Centres, there's a website where you can search for one nearby. They can charge anything they like for the information; a place in Wales charged my father £15 which he and I thought fair.
Regards
SS, November 2007