A little awkward but it did the job...
Went into contacts and changed the setting to VIEW/BY CATEGORY.
I then went to the column headings, left-clicked and then selected FIELD CHOOSER. I then added the 'Private' field.
I then expanded each of my 'personal' categories and worked my way down the screen manually ticking the Pirvate checkbox.
It wasn't quite the bulk-edit solution I was after, but it was certainly quicker than opening up each individual contact file in turn.