I thought I was not going to finish this one because the clues were already complex enough without the added burden of switching words. A breakthrough with first one and then the other 12-letter word enabled me to forge ahead with both sides of the grid, after which progress was steady, with a nice PDM to round it off.
I am dismayed, however, to learn that there's an even bigger hill to climb (spoiler not intended) with 'Puzzle Book', which I have yet to begin. If anyone is mystified, it's Ferret's D-graded contribution to this month's Magpie.