I followed 'Dragon's' instructions below for resetting my alarm after a power failure but it still didn't allow me to silence the alarm. Section 7 did not seem to apply because I have no SD display on my model. Dragon wrote:

You need the power back on to do this otherwise it wont work!
1. Open the Accenta 8 mini box by undoing the 2 screws.
2. Disconnect one lead to the battery.
3. Look for the mains power supply cord; on the RHS. Below this at the bottom right hand corner is a small mains transformer covered by a lightning sign. Above this and coming out of it are two white secondary wires leading to two terminals of a 4 terminal choc block. Disconnect one of these two white leads and re-connect again. The alarm will stop for a moment and start again. Meanwhile the touch-pad should now come on and bleep.
4. Go to the touchpad and enter your code (the code which you use to set your alarm) – the alarm will now stop.
5. Reconnect the lead to the battery you disconnected in step 2.
6. Replace the cover of the box.

7. Disable the Speech Dialler beep.
The SD display will show “PLEASE RE-RECORD” and the unit will beep every 30 seconds. This indicates that the memory is blank ie all telephone numbers and messages are gone.
Press ENT key for 6 seconds – the beep will stop and the display will read “READY”.
You will then have to re enter the phone nos in the SD keypad – see SD instructions.

8 Turning off the blue flashing light.
After each event and resumption of normality, the blue outside light will keep flashing to indicate the alarm has been set off. To disable this:
Go to alarm touchpad. The DAY light will be off and only the POWER and TAMPER lights will be on. Press Reset to clear the indication.

Simon, November 2006

I forgot to add that replacing the cover of the box (or removing the cover) will also trigger off the alarm. Once you've followed 1 to 5 and go to replace the cover the alarm will be set off again - simply key in your code at the touchpad. This should silence the alarm.

Dragon, December 2006