Ok, so here's what happened...
I decided to back up all my photos anyway, as it's a while since I did it! I then deleted the iPhoto preference file as you described above and relaunched iPhoto.
To my alarm, it wouldn't load at all and told me that my iPhoto library was damaged. It was also giving other error and warning messages about iPhoto upgrades and compatitibility. You can imagine how glad I was that I'd done a back-up at this point!
Anyway, once I'd stopped panicking, I simply recovered the old iPhoto preference file from Trash and restored it to its location in the User Library Preferences folder (overwriting the new one that had been created). To my relief that worked and everything is now back to as it was.
So, no harm done, but of course I still have the same problem. Unless you or anyone else can suggest another fix, I will have to continue to what I do at the moment every time I want to import some photos... Launch iPhoto before I connect the camera, select the "When camera connects open iPhoto" option; quit iPhoto, then connect the camera. A pain - but it does allow me to import photos.
Thanks anyway for your suggestion.
Pete