In Dec 2006 I bought one (Kidde. model 900-0146) which is still going strong from B & Q. I recall it seemed a bit expensive compared to the ordinary smoke alarm but it does seem to be more complicated in the job it does. (If you see what I mean)
It takes three AA batteries (standard or re-chargeable) which last for ages.
This detector sniffs the air every 30 seconds.
I have had one on my kitchen ceiling for 4 years, Scotman - a little green light flashes every so often so you can see it is working and the battery has now ran out. Our make is EI and we bought it from B & Q.
As DEN says, the little green light flashes at regular intervals and it beeps when the batteries start to run out.
There is also a test button and a reset button.
It should be fixed near or in the same room as the gas boiler/heater and should be audible in the bedrooms.