Found instore at Tesco Hoover Building (West London).
Located opposite the England football shelves (left as you walk in) in the Electrical aisle.
Fire Alarm is on a top shelf, sel stating £0.56.
Many left on the shelf. The bottom shelf has an almost identical product but without the optical option & is priced at £12.44 so be careful...http://www.fireangel.co.uk/Smoke-Alarms/Optical-Smoke-Alarms/SO-610.aspx
- Gives you the exact lowdown.
Barcode: 0 816317 000714
Hope it helps..
Coincidently, I bumped into a fireman on the way out who advised of the free home-fire-health-check but laughed when I told him how much I had bought the alarms for..!