Imobiliser will not turn off using key fob

[Mike]

My 2001 Omega recently developed a problem in that I cannot unlock or lock the doors using the remote keyfob I have to use the key in the lock to open the door and the alarm sounds as soon as I do this - starting the engine stops the alarm- is the keyfob faulty? is the imobiliser faulty? is there anyway to disconnect the klaxon/horn? The keyfob has a new battery and when buttons pressed the lights come on OK