All done now after hour upon hour of trial and error for the main code. Then the same for the anagrammed code, which I suppose I didn't really need, but it's nice to tie up loose ends. That said, I still cannot find the two letter change in 8,28. Anyone have any ideas?
This was an extremely difficult puzzle but very satisfying when complete. Many thanks to the setter, an anagram of which is an old version of what most of us have in our heads right now.