A modern sci-fi retelling of
the Arthur/Riothamus theory, Steve White's novel Legacy (Baen, 1995)
where the heroes of the story are intercepted by the enemies of a race they
just met in deep space and as time-travellers from the future go back to
469CE to record (literally, thru video devices implanted in their brains)
the last days of High King Riothamus (Arthur) and his queen Gwenhwyvaer. Be
forewarned, however, that the Arthurian stuff doesn't happen until half-way
thru this rather grisly SF novel.