I picked up one of these instore last night and have just installed it. It's been on my 'to do' for two years! Hope it encourages at least one other person to do so by posting the deal!
OOS at Tesco direct and costs £9.75
£11.98 at B&Q
£14.99 at edf energy
£17.78 inc delivery at FireAngelDirect
Ionisation smoke alarm with Ultra bright escape light.
High Quality Ionisation Sensor
Complete with alkaline 9V battery
"Bluescape" escape light Ultra bright escape light will help to illuminate your escape route and provide a visual indication that the unit is in alarm
Dimensions (HxDia): 32x110mm
With test button
Low Profile design
details here... http://www.fireangel.co.uk/Smoke-Alarms/Ionisation-Smoke-Alarms/LSI%20601.aspx