@dennypage
Hello - I feel really thick now I've worked out what you meant! For anyone else as stupid as me, you need to do the following:
Download the 2 files upsmon and usbhid-ups which Dennypage has on the other thread circa post #104 - they download as .gz files which are compressed (like .zip, .rar etc)
In my case I was using a windows machine so I extracted them using Winrar and then copied them onto my pfSense machine - I'm sure there's a way to extract them in Linux etc but I'm not great at Linux!
STOP the UPS service in pfSense
I backed up the two existing files in their current directories (/usr/local/sbin/upsmon and /usr/local/libexec/nut/usbhid-ups) by adding .org as a suffix to their name then moved the new files into the directory
The permissions were wrong by default so both files need to be chmod 755 before the service can be started as per the last posts on the thread
You should see the following when you're done if you run the command: ls -l /usr/local/sbin/upsmon* /usr/local/libexec/nut/usbhid-ups*
-rwxr-xr-x 1 root wheel 333728 Jan 15 15:37 /usr/local/libexec/nut/usbhid-ups
-rwxr-xr-x 1 root wheel 287088 Nov 17 13:41 /usr/local/libexec/nut/usbhid-ups.org
-rwxr-xr-x 1 root wheel 87760 Jan 15 15:37 /usr/local/sbin/upsmon
-rwxr-xr-x 1 root wheel 68888 Nov 17 13:41 /usr/local/sbin/upsmon.org
After this it was just a case of restarting the nut service and it started perfectly.
I am guessing that pfSense will release a new version of these files in the coming weeks / months which will remove the need to do this, but in the mean time, I believe the above seems to fix the issue!
Thanks so much to @dennypage for his patience and support with this issue - I'm still learning all this stuff! đ