After a monumental struggle, and a rather fortuitous PDM wrt the ring clues, I now have a full grid - and for once am happy with all my answers. The clues are extremely difficult, with quite a few with not quite what I would call a precise definition - for example radial clue 2. However, the end game is baffling me. I have 12 occurrences of the interrupting letter in the "grid", 13 if you count a common one which I do not know if it should be in the grid (as this would not fit with 16 other answers) and a sequence of 6 letters that does not appear to help me. Off to do a bit of shopping for Crimbo, and will return broke, but hopefully refreshed, for a fresh look at this.