To answer my own question, there doesn't seem to be a way to do this the way I wanted. My original folder structure had the music divided up in alphabetical sub-folders like Artist A-F, Artist G-H etc. I copied this across to the new drive, changed the preferences to the new address, and did a search and replace on the Itunes Library.xml file so that it would preserve my file structure. Didn't work.
So I tried the recommended Apple method, which preserved the artist/album folder structure, but destroyed the levels which I'd created to break that up, so that I now have all my files transferred, have preserved my album cover and equaliser information, but now have a folder with 1300 subfolders in a single level. Not very satisfactory at all, but that seems to be all that I can do.