Name of book on arthurian legends

Question

I once read a book that had a aurtherian background. It was about a knight from his fellowship who now lived peacefully in an island. And then some raiders come in to the island and finally the knight who is one handed takes out his sword and goes to fight. When he does he feels the army behind him riding into the war just like the old days. It has an alternative take on legend of arthur that they were young and foolhardy. Tried to make too much too fast and failed. Written half as remembrances of the past half present time scenes. There was mentions of war cries and I remember the name Cei. The book was british. Anybody can remember what this book was ?