You might get better answers on such specifics in the LR forums. If you used the LR Beta, I could imagine any number of things interferring here, one possibly being corruption or incompatibilities in the catalog files or custom metadata fields preventing recognition and conversion. in these cases you probably will have to fiddle with teh pertinent prefs and XMP config, but it's realyl best to ask the people who know better than me...
Thanks very much for quick reply. I theorised that LR4 had trouble picking the right LR3 catalogue to upgrade as I have 3 listed, so I used LR4 to open the latest LR3 catalogue, then upgraded that as per LR4 prompt, and all my pictures were back agan!
Next problem was that I got a 'No image selected' message in Develop and no images in filmstrip: a Google search on that issue suggested simply restarting LR4 and indeed that sorted that issue!
So now seem to be up and running!
Thanks again for stepping in and helping.