SOLVED!
for anyone else getting the same FAILED error for the board when trying to compile I finally got it working after changing folder permissions.
Firstly I extracted the contents of the Marlin zip file (I ended up using the newest Fysetc branch files) and I saved them all in the root of my c: drive so the folder structure was C:\marlin\platforimio.ini etc etc
I found that the issue was with the C:\marlin.piolibdeps folder. I had firstly claim this folder as my own using the Advanced button on the Security tab of the properties pop up for that folder (right click properties) and then removed all other permissions and gave myself full permissions for the folder.
After doing this the compiler would run. I now have a working install or custom Marlin with 4x4 Manual Mesh Bed Leveling and Eprom SD emulation working well and saving settings that are changed on the printer directly. And I am printing via Octoprint. Very happy (finally) and very quiet, apart from fans, but they are on order.