Thank you very much for your help Aleksey and Digman.
I tried your suggestions and they worked great.
However, I have another question as a follow up based on the revised workflow you described. Unfortunately, I do not have screen prints at the moment (sorry, different computer) but I will describe the issue.
Steps 1-3 are fine.
Step 4: when importing the Retopo Mesh, 3DC asks if the mesh should be snapped.
If I click YES, the retopo mesh (as it suggests!) snaps to the new geometry and I can proceed with the rest of the workflow.
However, the retopo mesh (when snapped) creates a number of polygons that are not in the original model.
If I click NO, the mesh aligns well to the original geometry and does not snap to the sculpted areas. By clicking NO to Snap, I do not get any artefacts. However, this means I cannot apply the diffuse map texture to the sculpted areas.
My thoughts are at this stage that I may need to Autopo, export, import, retopo, merge and paint. However, I guess I am asking if I need to go through this convoluted workflow or do you have a more elegant solution. I cannot discount the NOOB factor so I may be making a simple mistake.
Thanks once again for your help.
Ash