If the unset light is flashing and you cant set the alarm, its simply that the recharcable battery in the main control box is knackered (usually only lasts 5 years). You can buy a new one for about £15. And easy to change with just 2 terminals to attach. If you cant put it in engineers mode, the alarm will probably sound for tamper. But continue when finished you can reset.
Andy, September 2008