Well I solved my issue, although I'm not sure what the difference is:
I copied every dll from my qt directory into both ./ and ./platforms of my application directory.
The application got past the error, but then crashed.
VERSION.dll was causing the crash (noted in dependency walker), so I removed it from both places.
The Application started up, so I systematically removed all unneeded dll's.
This got me back to the same state I had originally.
I then uninstalled my application and re-installed (with only the ./platforms/qwindows.dll file remaining), application works correctly.
So all I can assume is that I had an incorrect version of qwindows.dll in the platforms directory.