A good basic CO alarm of a reasonable size and looks, with the added advantage that you can change the batteries. It is supposed to let you know when they need changing but I haven't experienced this yet as I have only just installed it.
BUT, make sure that you get a working one in the first place. I ordered two when our old detectors, originally installed by Lifeline, who failed to service/replace them, and which the county council started charging for, came to the end of their life anyway. I did some sums and quickly proved it was cheaper to buy our own.
However, the first one I installed was OK for a couple of days and then started going off randomly for no reason, so I swapped it for the second one and that seems fine. I requested a replacement but the suppliers quickly gave me a refund as they were out of stock only a few weeks ago and didn't know when they would get some more. Net result - we now only have one detector instead of two!
For information, these detectors do go on a wall but, unlike some, they have two screw fixings, not one. Our old ones only hung on one screw and I'd hoped these would too, which would save messing up new decoration, etc., but no, two screws.
And finally a couple of words of warning. Firstly ALL these CO detector/alarms have a life span of 5 - 7 years after which they must be replaced. Most warn you. It is nothing to do with the brand; it is the cell technology.
Secondly, reading some of the other reviews, some people seem to think that it is alright to just stick these devices on a shelf or sideboard or wherever. It is not. These devices must be placed at an appropriate height above the appliance which they are monitoring. Read and follow the instructions.