More exploration of Devonthink and note taking, I discover that all the notes/annotation made on a pdf document in skim can be imported into skim as .rtf notes.
There is a script which allows the importing of the notes from Skim.
Skim Script here.
The script imports the Skim notes into a new group (folder) with the title of the skim document as the folder name. Place the downloaded script is placed into the Devonthink scripts folder. Update the scripts folder so it shows and then import. The document in skim must be open for this to function.
Very useful, although I will now have to be more judicious with my annotations of pdf documents in Skim as this will produce lots of individual .rtf files (one for every highlight, underline, note etc).