Why on earth would it be looking for Python and a C++ compiler? Well it turns out one of the node-sass dependencies is node-gyp which needs to be built before it can be run and this uses Python 2.7 to be installed on the system. Also, node-sass is based upon libsass which requires a C++ compiler to be built. So now I know.
However, installing Python 2.7 did not seem to solve the initial issue, even with the PATH set in the environment variables. And the C++ task error was pointing me to install up .Net 2.0 SDK and I could do without that monumental faff.
Luckily, the solution came in a package called windows-build-tools and this took care of both issues in a single shot. Blammo!