We needed to put a CO sensor and smoke alarm in our holiday apartment in Spain so I got sent off to buy one. Tesco seem to getting rid of stock as I picked up.
FireAngel Toast Proof Smoke Alarm - £4.48 (b&q = £8.98)
FireAngel Digital Carbon Monoxide Poisonous Gas Alarm - £14.94 (b&q = £29.98)
Not the most exciting of purchases, but they may just save your life.