Thanks for answering all my points Box .
I had managed to use the method suggested by Dukejazz of copy and then scaling (that was before I had seen your first post). The next problem was that I needed to add a lip all the way round the inside and ultimately fuse the two together. However, What I have created just doesn't seem to want to be a solid. I have Softened/smoothed all the lines and it seems to select as one surface now. There seems to be a surface closing the gap between the two shells. I have selected the whole thing and made it a Group but still get the message about one or more components not being solid.
Any tips or ideas on what might be going on here?
Think I might go back a few steps and re draw the shell with simplified curves as per your suggestion and see if I can get it looking a bit better. Be nice to smooth out some of those ridges...