Every time I try to install the diplomacy mod (using it's installer) it comes up with an error saying that my version of the game is not recognised and the mod will most likely not work with it. The Diplomacy module does not appear after installation.

How do I get Diplomacy mod to work with gog.com version of M+B Warband?

I have exactly the same problem, yet everywhere I look for answers, I find nothing but steam. Literally, steam; it's as though no one plays the GOG version or something...

I tried locating the file for Warband, but I could only find the setup wizard and uninstall aforementioned. I'm wandering in the dark as much as you... I do remember something being said a while back about having to have the version from either steam or the producers, but i'm unsure as to how reliable that information is.

I get that error even with the Steam version. Make sure the path in the installer is correct and it should work.

You need to download the Mod.
I got mine from nexusmods.
Find a program to extract the Mod.
Open GOG.
click on installed games
right click Mount and blade warband
Click mange installation > Show folder
Find the Folder Modules
and paste that extracted folder in there
When you start up game it should have a little drop down menu to select diplomacy instead of native version of game