To add to what Ron has already posted there is another issue. The siren itself has an internal battery which eventually dies then leaks over the siren circuit board causing the same issues. Either replace the whole siren module or take it apart and replace the batteries. I did this a few months ago and is still working great. Total cost about £12 and a few hours work. There is a thread somewhere about it which I contributed to so have a search ( can't do links whilst im using the mobile to post).
Matt