Well, with a little help from my friend I am more or less finished, NW corner just dragging a bit but just love the theme and presume it is the second 11 letter down clue we highlight. Such a clever construction which would have been totally beyond the feeble grey matter without the nudge, but such enjoyable clues. And NO Maths!!!!!!
The source of the 13a is 1a, so R4E should be highlighted. I did wonder why DUO and ANI were not included as down entries - perhaps we shall never know !