I've noticed that in iPhoto Library / show packages / there are two folders, named Modified and Originals. These appear to show every image I've worked on since day 01. Can I safely delete their contents? does anyone know if this will help.
Deleting these folders will delete " every image you've worked on since day 01." On no account delete them - this is where iPhoto stores your original images and the edited versions.
To amplify what Leonie says:
Don't change anything in the iPhoto Library Folder via the Finder or any other application. iPhoto depends on the structure as well as the contents of this folder. Moving things, renaming things, deleting them or otherwise making changes will prevent iPhoto from working and could even cause you to damage or lose your photos.