Earlier in this thread someone suggested "blowing" in the detection chamber.
Technically, at least here in the U.S., it's called the 'target'. It's a smallish (maybe 2 inches in diameter) shiny cup shaped device into which the particles of smoke collect to set off the alarm. These need a regular cleaning, but blowing doesn't help much since the household dust particles that collect over time are sticky with various and sundry by products of cooking, etc.
Since you detector is powered both by a standby battery (I don't think that's your problem) as well as house current, you'll need to find the appropriate circuit breaker (hopefully it's labeled), open it then proceed back to the device. The top should just pop off with a little pressure. The 'target' will be quite visible and recognizable, even from my less than informative description. Take a tissue and wipe it out a couple of times (do not wet it with your tongue first, however). Replace the top, reset the circuit breaker, et voila. (If this doesn't cure the problem (I'll be out of the country the next few weeks and so unavailable for any snideness).