YES! I got it!! The issue was that steam version of Civ 4 BtS doesn't put much in the windows registry. Even the disc version doesn't create a BtS reg entry, just a Civ 4 complete one. However the BtS patch does create a BtS reg entry with a language setting, install directory & patch version number. Through trial and error I've found that the language & version number reg entries have no impact on the B5 mod but the install directory record reg entry does. Without it the B5 mod won't work. Oddly enough if the directory address is wrong it still works. I've changed it to the default steam install folder however you could put C:\HairyBalls and it'll still work lol.
Here's the text if you wish to paste it into notepad and save it as a .reg file. Or you if want it even easier I've created this one attached. Just unzip the reg file and double click on it and it'll insert the below entry into your windows registry. Run the B5 mod and it should be all good now!

(note this has only been tested in win 7 however it 'should' be fine in win 8 & 10).
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Firaxis Games\Sid Meier's Civilization 4 - Beyond the Sword]
"INSTALLDIR"="C:\\Program Files (x86)\\Steam\\SteamApps\\common\\Sid Meier's Civilization IV Beyond the Sword\\Beyond the Sword"
I might write a proper install guide in my next post and also include a copy of the guide as well as this reg file in the main mod zip file download since PsiCorps has kindly given me admin access to the B5 sourceforge page. This should vastly reduce people having problems running the mod.