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.

Legacy by Steve White (Paperback - Feb 1, 1995