Failed to load platform plugin "windows"

After having run my application successfully in Qt4.8 for a long time, I'm now trying to deploy it in QT5.1.0. Everything has built cleanly, Dependency Walker confirms that all of my dependencies (Qt5Cored.dll etc. - the 'd' because I'm initially trying to install a debug version) are present and correct, and I've copied the whole of the 'platforms' directory from the 'plugins' folder into the same directory as my executable. However, when I try to start the program, I obtain the error message:

"Failed to load platform plugin "windows". Available platforms are:
minimal
offscreen
windows"

So it seems to want to have it both ways - it can't find "windows", and yet "windows" is one of the ones that it can find. Either way the program won't start.

What can be causing this?

OK, I found the solution here. It turns out that I needed to have libEGLd.dll present in my install directory as well, even though Dependency Walker hadn't picked this up.