Thank you very much for your help!
Summary of my problems (for reference by others):
I never did manage to successfully download from SourceForge. When issuing the command:
git clone git://opencpn.git.sourceforge.net/gitroot/opencpn/opencpn
I got the reply:
.git.sourceforge.net[0: 220.127.116.11]: errno=Connection timed out
fatal: unable to connect a socket (Connection timed out)
So I am using the version from the link you gave me: https://github.com/nohal/OpenCPN
Then, when I tried to run CMake from the Visual Studio 2008 command line, like this:
cmake -G "Visual Studio 9 2008" ..
I got the error:
CMake Error: Could not create named generator
Visual Studio 9 2008
(I'd managed to run this command successfully before so don't know why it's stopped working now, but anyway...)
I decided to run the CMake GUI. But after following the OpenCPN
build instructions for the CMake GUI I got sa big red error saying "Could NOT find wxWidgets (missing: wxWidgets_FOUND)
It turns out I'd simply omitted to add the correct paths to the wxWidgets vc_dll in the "name" and "value" section, in the middle of the CMake GUI.
After that it all configured, generated and built (in VC 2008 Express) as per the instructions.
Thanks again for the help Pavel.