I accidentally deleted the folder with the DNG files after importing files and editing them in Adobe Lightroom. Played around a bit and managed to re-generate them without losing my editing and tagging, using the following procedure:
1) Select all the raw photos for import (again)
2) Tick (if not already ticked) “Do not import suspected duplicates”
3) The dimmed thumbnails are the ones you want to import as those are the ones you imported last time
4) Un-tick all
5) Un-tick “Do not import suspected duplicates”
6) Tick the ones that were dimmed down
7) Un-tick “Add to collection”
8) Import
That re-generated the DNGs
I then generated a new web-catalogue and watched the file sizes as it built the files and the were all 30-40% bigger, so it definitely made a difference.