Could be a faulty switch, need to put alarm into test mode and then go around and see which switch doesn't work.

Been a long time but if I remember correctly press the bonnet switch 8-10 times then you will hear the alarm horn beep to show its in test mode.

Then go around open/ close each door and work each lock and it should beep every time a good switch is actuated, if it doesn't beep there is your problem.

The door lock barrels themselves have switches too

More info

http://www.rs2000-16v.co.uk/newsitefiles/alarm.htm