Kidde 900-0230 BSI Battery Premium Range Carbon Monoxide Alarm with Digital Display £15.29 @ Amazon.co.uk
Cheapest I could find.
We had our annual boiler inspection from british gas yesterday and he recommended us getting a couple as our flue is quite big and runs through the middle of our house. He then tried selling us 2 of british gas own models which looked quite basic and cost £20 each.
These look much better and cheaper!