Thanks for your input. I tried it this way - I parented the spline curve to the Main Ctrl. I didn't make a cluster from the last point on the spline curve, so I did this and also parented this to the lower back controller. This appeared to work but then I had the whole of the bottom part of the spine rotating including the root node. I'm not sure if this is technically or anatomically correct. So to stop the root node from rotating aswell, I made a locator and constrained the bottom cluster to the locator which seems to lock the root node in place. See pic attached with character moved forward. Fingers crossed but I think everything is moving ok.
BTW, I looked at your website and thought your work was fantastic. Those girl animations were brilliant. How did you get her hair to flow in that way? Did you use 3DS Max for your work?