Well I've finally got DX12 to start in SLI now without it switching back to the Windows desktop as before I had to disable SLI to get DX12 to work but now thanks to the tip sanoj52 said about the page file It now runs with SLI enabled. I had my page file disabled when I got my 16GB of ram not too long ago but when I set it back to system managed I could now run DX12 without having to disable SLI.
Finally got it past the initializing screen by deleting the settings.ini that The_Gear mentioned and also disabling SLI. If I enable SLI again and try and load through DX12 I get the usual drop back to Windows desktop, though before the last update I didn't need to do this. Well at least now I have the game running in DX12 again which I'm happy with.
Just thought I'd chime in here and say I'm also not getting the DX12 version to run, I just get the title screen come up and it saying "Initializing" followed by my desktop. This only started after the 16th of September update, a shame as I only purchased the game a few days before this and it loaded up fine then. Anybody have any ideas?