I now have an answer from Technical Support with reference to the other conditions of the 'Resume' function.
As said before, Resume does not work on BD's when the source is an optical drive - Cyberlink are working on this.
Resume does work for DVD's when the source is an optical drive.
Resume does not work for DVD's when the source is a HDD.
If the Movie Library has the HDD and directory identified, resume works, however, if you have more than on HDD in the list and/or more than one directory, all the titles are pulled into a single screen display with no reference to their parent source. In addition if a DVD/BD title has multiple discs, the Movie Library displays Disc 1, Disc 2 etc but does not show the title folder; all very confusing.
I have suggested to Cyberlink, it would be more user friendly to reinstate the resume function as it has been in previous versions for HDD sources and make the Movie Library reflect the source structure. They say the proposal has been added to the list of wants, so I assume, no change in the near future.