How can I remove a tamper lockout on my Optima XM6 keypad ?

Over recent years our alarm has sounded whenever mains power is restored after a powercut. Although a bit of a pain in the night, this never caused a serious problem until the last time it happened, when the tamper light stayed on and the system would not reset. The battery fuse had blown on this occasion, and I've replaced this, and tested the 12V battery, which seems to be fine. Disconnecting and then reconnecting the mains and battery power sets the alarm off, as per the powercut, but then no change to the tamper status/keypad lockout. The PCB looks fine and none of the wiring through the house has been disturbed recently. Bridging the tamper loop (terminals 15 &16) makes no difference to any of this.

I'm grateful for any helpful advice how to restore normal alarm function.