Well, it's certainly not that hard, but try as I might, I can't make my third substitution, despite reading everything I can find on the context and trying out various possible lights. Part of my trouble is that my two changes alter ten of 12 letters in the entries, which on a strict reading of the blurb means I have to change seven more, which rather limits the number of places I can work on. I'm probably just being dense, but a prod in the right direction might enable me to avoid a sleepless night going round and round the grid.