yep you have answerd your own question change the battery and if it works great but i do know this for a fact if the unset light is countinusely flashing then in the big main panel look where the mains supply is fitted and remove the black pull out fuse and see if it has gone if it has in the panel theres normally spair fuses so replace it if it has gone 
remember still change the battery 
get back to me if it works well 
Death
                
 Death
                
 May 2010