I thought the key had a chip in to get round that. Unlock the car with the key and the alarm will sound. Put the key in the ignition and turn it to position 2 (ignition) and it should stop the alarm and allow the engine to start. I know this works on Vauxhall and Citreon systems, not sure about the quality Rover system though.
If you know your security code, instructions in your handbook will tell you how to "re-mobilise" your car, you use the key, in the door lock, turning left and right to the numbers on your security code, bit like a safe.
That sounds more like Rover to me. You need to know security codes AND how to work a safe. I don't know what my radio code is yet alone the security code for the alarm They can't make it easy can they.