Thanks for all the useful replies. I'm glad I came across this site this morning. I've downloaded Irfanview - shame I didn't do it ages ago - it could have saved me many hours of batch processing. I'd never really understood it before but Irfanview makes it really easy! I think it will be easiest to change the file names of the new files from the K100D Super (see, I put the full name this time) rather than going back through the 'old' files.
Just to clarify how I've been saving my files...
I open the original, make any changes to sharpening, colour balance, etc, then change the image size to 10x15cm @500dpi 'save as' "IMGP0000 title" (which gives a file size very similar to the original), then change the resolution to 72dpi and save as "IMGP0000 small title" to give a version to view on screen at 10x15cm size and which can easily be emailed. These three versions are all saved into folders of 1000 images (or 3000 files). They are all searchable by name = image number or by 'date modified' - or, of course one can search by a string using the usual search facility in Explorer.
Thus the original file is only opened once and saved unmodified; there's a copy at 'high res' and a copy at 'low res'.
You can see some of them at
Naturepix - Photo Gallery Index...
I've also had a chance to check out the bit about what happens when you put in a new card - and, yes, it simply carries on with the numbering sequence from where it left off with the previous card.
Thanks for your help.
Happy snapping.
Simon