I gave Marvelous Designer’s retopology tool another go and… god… it is such an awful tool.
I mean, I get that this isn’t really their specialty, but still.
The pictures I’ve included, are the results, so far of my attempts using RetopoFlow (Try it out, it is quite good https://github.com/CGCookie/retopoflow). Progress is slow, but I feel that the result is a lot smoother than when I did this completely manually.
The surface constrained smoothing tool is especially useful… (when it doesn’t snap to the underside of the original model)