Alarm System not arming

I have a 2008 9-3 Aero Convertible, recently I noticed that the red indicator lamp on the dash has stopped blinking and the alarm doesn't sound. The car still locks and the lights flash but there's no response from the red light and it stays off. I've tried a reset by hitting the unlock on the remote five times but that didn't help. I'm wondering if it's the battery in the alarm system and I'm wondering how long they last, whether it's a fix the dealer must do or whether it's a DIY project.

Any advice would be appreciated