I've put up a file called TempLS.pdf (about 80 kb) on the document repository. It sets out the least squares theory, and goes through the analysis code explaining the implementation.
It may already be a little bit obsolete. It's likely that V1.4 will solve the design system by the conjugate gradient method. The matrix is well conditioned, and the method seems reliable, and at least five times faster, making inversion a matter of 10-20 seconds.