It's an 85 DB Carbon Monoxide Alarm.
Same one is £26 at B&Q.
This one is Mains Powered so no batteries to run out.
What more can I say but this thing can save lives.
It's ebay's deal of the day.
Kidde Carbon Monoxide Alarm 85 Decibel Mains Powered
* 230v Mains Powered, However Has A Battery (Included) Back Up For Power Cut Situations
* Peak Level Memory, Test button and LED allow simple verification of alarm operation.
* Electrochemical Technology, Flexible mounting positions
* Loud 85 decibel alarm - Alerts you of danger whilst you sleep.
* This product is suitable for domestic use.
* BS EN 50291 Approved, Comes With 5 Year Manufacturers Warranty