Firstly I'd like to thank all those who contributed to the original post about the classification of modern and historical books - you certainly gave me some food for thought.
As well as on Pen And Paper, we also had this discussion on Book Blogs where several different theories were put forward. My thanks to you all but especially to Jaime Huff over at REVENGE OF THE BOOK NERDS (as well as Book Blogs) who discussed the matter at some length and pointed me towards the following sites which help to clarify the matter.
"Historical fiction is a sub-genre of fiction that often portrays fictional accounts or dramatisation of historical figures or events. Writers of stories in this genre, while penning fiction, nominally attempt to capture the spirit, manners and social conditions of the person or time(s) presented in the story, with due attention paid to period detail and fidelity.
"Historical fiction presents readers with a story that takes place during a significant event in that period. Historical fiction often presents actual events from the point of view of people living in that time period."
* The Historical Novel Society.
"To be deemed historical (in our sense), a novel must have been written at least fifty years after the events described, or have been written by someone who was not alive at the time of those events (who therefore approaches them only in research)".
Petty Witter says: Judging by the definitions supplied by these sites, I think I'll go back and re-classify the three books (Guernica by Dave Boling, The Return by Victoria Hislop and Hamers War by Francis Cottam) mentioned in my original post as it would seem they should indeed be classified as historical rather than modern fiction.
P.S. Having gone back and re-classified those three books, I found several others that really ought to be classed as historical rather than modern fiction - it's amazing how these things snowball.