cmk - by my reckoning, the cryptic part of 27ac will give you a word - not a real one - that would synch with the answer to 21dn. You modify one letter of the answer to 27ac to give a real word. As far as I can see, the letter to be changed can only be the one that checks with 21dn, and that's where it goes wrong. All the answers in the grid, apart from the Playfair ones, are real words. Of course, I may well have got this completely wrong, but I can't see where.
Hi ixion. Yes, it's a fascinating puzzle. I've got to the hard bit now - trying to work out the Playfair square. I've got no idea just yet.