Hi @CampbellDave; welcome to our Community!
When you wish to re-install the desktop app and you do not want to re-download all of your files from the server, you need to choose the "Advanced" setup option during the installation process.
Since you've already attempted to set this up, at this point, Dave, I'd suggest unlinking your account through the app's preferences (or 'options' if you're using the latest best version of our desktop client), deleting the newly created (empty) Dropbox folder from your E drive and then, upon re-linking your account, choose the 'Advanced Settings' option to point the Dropbox folder to your existing one.
For example, if your old Dropbox folder was added to: D:\Desktop\Dropbox you would need to choose D:\Desktop and the client will try and create a new Dropbox folder.
Since you'll be re-connecting with the same Dropbox account as before, Dropbox will detect the existing folder and will then prompt you to merge the Dropbox folder. Upon merging the desktop version of Dropbox will check the existing files and the server versions and update accordingly.
Let it sync those changes until it shows 'Up to date' within your system tray, and you should be good to go!
I hope this helps and please keep me posted!