[eluser]peter_k[/eluser]
Kinda Resolved - but I don't understand why ....
I uploaded a fresh copy of CI and the uploader worked just fine. So one-by-one I added my files. After much trial and error, I discovered that uploading my own config.php file caused the uploader to stop working. In my config file I had changed just one setting - the base_url. I'm sure I had it set correctly as all site files loaded as expected. However, when I set this to blank as was in the fresh copy of config.php, everything works perfectly.
I then tested a bit further to see was this causing the problem I mentioned earlier, that (in my previous installation) using the form helper to construct a form caused no data to be posted. And that turned out to be the case.
Conclusion - for my installation
With $config['base_url'] set to anything except blank,
1. $_FILES never gets any value from a form
2. a form created using the form_open helper function will not post anything to the $_POST array
I can't explain this at all, but am very happy to have discovered the cause of the strange CI behaviour.