How to fix my Texecom Veritas Home Alarm Power issue?

Hi - I have a Texecom Veritas 8 system. There was a power cut while I was not at home and it has never recovered properly. Power light at keypad was flashing and LED display was visible but not illuminated as per usual.. I suspected a faulty system battery and bought a replacement of same spec as the old one. I swapped the battery and it seemed to fix the issue.
Then I noticed one of the PIR detectors was no longer working. It still showed its red light when motion was detected but it did not chime when triggered and did not show up in the latch test I did. There were no error messages on the keypad to suggest it was faulty. I checked continuity on the cables between the PIR and the PCB and these were fine.
I then thought I would move the wires at the PCB (pair 3) to a different set of terminal posts (pair 4) and moved the existing pair 4 resistor to pair 3.
This made no difference except that the keypad reverted to flashing the power light. The Control panel power light also flashed green. This suggests to me that the issue is one of mains power getting to the keypad. However when I disconnect the battery and restart the system the mains power gets through to the keypad and the power light remains lit. When I downed the power to hook up the battery and restart it it reverts back to flashing green light at Keypad and control unit - suggesting mains power not getting through and that it is running on battery only. I'm wondering why the introduction of the battery is stopping mains power reaching the keypad. Unless it's an intermittent random fault that needs some more testing I'm stumped. Fuse covering the battery is fine. Other fuses are fixed/built in to the pcb so not changeable. Anyone have any idea why the battery is blocking mains power when both work independently of each other?