Don't worry Ray, I had a brainwave that seemed to make sense, and it worked!
I expected to have to create a session and populate it somehow, and then I remembered that there's a load of server variable options with the upload file field that I was able to choose from. I just chose the Server File Name option and that accounts for the incrementation if a duplicate file name is uploaded... who knew?
And, as I am typing this, it has just occurred to me that I have added this post to the wrong thread, so please move or delete as appropriate.
Thanks anyway.