Optima plus 2 - Alarm fault?

I have a optima plus2 alarm that goes straight into alarm sounding when the electric power is interupted.
I have installed a new 12v battery.
I cannot silence the alarm as there is no display leds at at all on the control panel and no sound from inside only the outside bell sounding. It does silence after the 10 min period. When power is re-instated the alarm sounds again and the display lights so i can then silence the system.
If i need the electric off for any maintenance i now have a neighbourhood problem.
Any advice please as i could repair it myself with help and advice.
Thanks