Not 1 but 2 Carbon Monoxide Detectors for £21.43 Delivered (£10.72 each). To get them for this price you need to also buy a Tesco mobile Sim card (from £2.47) which gives free delivery on your whole order (normally £4.95).
Buying 2 enable you to use the code TDX-CT32K7 which gives you £5 off a £20 spend. You could always buy 1 detector and buy other stuff to take your order over £20. I've no idea when this code expires.