Optima XM6 Key Panel unresponsive?

I have just moved into a new property which has the above named alarm, I have spoken to the previous owner who has never used the alam but had the code. He had taken the mains fuse out and the battery from the control panel.

For peace of mind I plugged everything back in and the alarm sounded as expected. I punched in the code provided and the internal bell went off showing only the tamper and the power red LED's showing. However the outside siren doesnt turn off. Once the code was entered the keypad then becomes disabled. I have checked the two internal fuses both seem ok.

Any ideas on how to reset the alarm system?