I may have told you wrong. The command would not need to include the passcode as that is appended internally from the binding. Try the following:
sendCommand(SEND_DSC_ALARM_COMMAND, “040,1”)
This means the passcode needs to be included in the configuration.
Sorry about the misinformation.
That’s no problem. It may well help others trying to learn how it is done 
I’m not out of the woods yet though, This is the log output but the system does not disarm…
2019-01-16 20:08:00.429 [ome.event.ItemCommandEvent] - Item 'SEND_DSC_ALARM_COMMAND' received command 040,1
==> /var/log/openhab2/openhab.log <==
2019-01-16 20:08:00.430 [INFO ] [se.smarthome.model.script.Alarm test] - ---------------> Alarm off
==> /var/log/openhab2/events.log <==
2019-01-16 20:08:00.439 [nt.ItemStatePredictedEvent] - SEND_DSC_ALARM_COMMAND predicted to become 040,1
2019-01-16 20:08:00.446 [vent.ItemStateChangedEvent] - SEND_DSC_ALARM_COMMAND changed from 500: A command has been received successfully. to 040,1
The command is (and the actual code is stored in the binding config through PaperUI):
sendCommand(SEND_DSC_ALARM_COMMAND, "040,1")
Sending the disarm through HabPanel works fine, so I’m just stuck on this issue.
I don’t see your definition for the SEND_DSC_ALARM_COMMAND item in the item file. Is that defined elsewhere? It doesn’t seem to be linked to the correct channel.
What does the log look like for that?
Excellent spot. The problem was a mistake in my item definition (as you correctly pointed out it was not linked to the correct channel). Now it works fine.
Thanks again for your help