Thanks, I realise where I was going wrong.
I have another problem, this time with the external sounder.
The cable supplying the existing sounder has 4 cores, previously 2 used for +ve and -ve, 2 used for the tamper circuit.
I have bought a kinetic AZ-Tech AZ-220 external sounder, that appears to require 5 cores for it to function correctly.
I was thinking of not using the tamper function (the sounder is pretty difficult to reach), shorting the switch on the sounder and tamper circuit in the control panel.
Any suggestions? Running in another cable is not an option.
Thanks!