I worked out a way of doing this. I created a Calculated Field that had a FALSE value. I then added this to the detail of the sheet in question. When creating the Action I set the Target Filter Source to this FALSE field and set the Target to the CheckedIn field.
I did the opposite for the CheckedOut count, TRUE calculated field, as I wanted a count of what had been checked out, and create another action pointing only to the checked out sheet in the Source Sheets.
Remember also to set the Clearing the Selection Will field to Show All Values for each action.