Solved. Solution here for others...

This line is needed in the pro-file to pick up the 64 bit libs...

win32:LIBS *= -L"C:/Program Files/Microsoft SDKs/Windows/v7.1/Lib/x64"

I had this previously (see pro file in first page), but also had an extra non-64 bit directory from the SDK. It started to build when I removed the other line.

Also had problems with boost "Macro argument mismatch bug" errors. Had to put this line in the pro file to solve this...

QMAKE_COMPILER_DEFINES = _WIN32

More here...

http://stackoverflow.com/questions/1...t-mismatch-bug