Ash Mill

Hi all, We inherited this alarm from the previous occupants. It behaved itself for a good eight years or so but recently it's been beeping every half hour or so. If we key in a code we have it usually shuts up for another thirty minutes before starting again. If we don't do anything the beeping will become more frequent until its going every few minutes. We seem to be able to access the full menu, but none of the immediately obvious menu options seem to work. I can't make head nor tail of the instruction manual. Can anyone help? Is there a way to get it back to its old self? Thanks!