After a bit more digging, learning and receiving a wholly unhelpful response from Ucancam support, I have more details regarding my problem. My next port of call was to uninstall and reinstall the software. I did so making sure to backup the AppData folder containing our tool library, Post Processor *.set files, and presumably other configuration files. On re-installation I have discovered that no post .set files were installed and had to grab the old Generic CNC file and put in the newly minted AppData folder. However the exact same problem persists. I have drawn a 20mm x 20mm square and output it as a .nc file. Upon bringing it into Mach3 and reading through the G-Code I can see that the instructions in the code state 42.2 as the sides of said square - presumably this is in millimeters as that is what we operate in and what the resulting output measures as when we mill with the file. I've had a look at the post processor file in a text editor, but am not entirely sure what I am looking for and it seems to match what I would expect from examples of these files I've found online.