Thanks for the reply aiharkness, and thanks Dubdub for moving the post, I wasn't too sure where to put it originally.
Well after making sure the USB cable and USB ports I use work properly, I've just this minute finished a Desktop Software upgrade (to 184.108.40.206) which seemed to work fine. Then I tried the software updates again, that had been giving the "fatal error" message, they now seem to have updated and installed fine.
As for installing/uploading a ringtone to the phone, after the update and upgrades it now appears that I can open the "J disk" and see the folders files etc.
So it looks like everything is now working okay again.
Once again, many thanks for the help.