If your willing to try and change batteries then you could disconnect the horn and cover the keypad to stop the tone
It will go into tamper and then you will have to wait for engineer anyway or default it and reprogram
Being a 264 I assume has plenty detectors / circuits being used , or you have no idea on that either?
Call another engineer ? Either way so done has to pay
Sounds like usual they don't want to pay lol
Unbranded and they dear , security warehouse decent brand and cheaper but op is worrying about returns incase panel pops
https://www.screwfix.com/p/sealed-lead-acid-battery-12v-7-0ah/38315
Do a drawing or upload a picture of what you have done
If the relay is energising then must be the mains side of the relay you have done something wrong
You would it , you still have a timed exits or instant set , or contact set depending on how everything is setup
I don't really know the panel well just entertainment for you while everyone is having tea break