Whistler, Jamesm, many thanks for your help. I had made things impossibly difficult for myself by making an error in 2 down. Once I had resolved that, replacing the letters in rows 5 and 6 was straightforward. All I need to do now, is work out what to enter in circles 1, 2 and 5. I think I know what may go in circle 5 to provide the "sole letter", but I do not see how it relates to the clashes in 5 down. As for the 6/11 clashes, I haven't a clue.