Thank you for all your kind replies. We are obviously completely devastated by what has happened and it will take some time to get over the pain of our loss.
Lesley, we followed your experiences closely as we recognised the similarities in behaviour, and feared the same ending. We took comfort that we weren't alone, that it wasn't our fault and that we did everything possible to help him, as you did. These problems are probably more common than realised and unfortunately not well researched or understood.
Thanks everyone.