The immobilizer is an integral part of the ignition system and as you probably appreciate if broken (most usually by inept thieves) is an expensive replacement system.
The Ariel which "recognizes" your key to the control unit however is just a coil of wire encased in a plastic ring around the ignition switch body (also often damaged by inept thieves or owners).
Check this part first.
Taggleflange
January 2009