Simon, what I think Mark is referring to is that there are two ways to import files which have been shot as raw and JPG. These depend upon the setting of
"Treat JPEG files next to raw files as separate photos", which is accessed thru Edit > Preferences > Import tab.
If this box is checked, then the raw and JPG files are imported as completely separate image files. If the box is unchecked, the raw file will be imported, and the JPG file will be treated as an attached sidecar file, for the purposes of file management, moving, etc.
If your images display a file type of 'CR2 + JPG' , or 'NEF + JPG' etc. then you have imported the image with the separate files box unchecked. You cannot use LR to manipulate the JPG in that case. You would have to 'Remove' the raw + JPG pair, and reimport as separate files to gain access to the JPG.
I hope I'm understanding your situation correctly.