Lovely construction - thank you Vainglorious. 2 slight quibbles, just in case anyone else has them too.
First is with the preamble. My answer to 18a is in Chambers. My answer to 14a, however, is not. Has there been a misprint?
Also, I spent a long time on the SW corner because I had the wrong answer for 40a. I still think my initial answer ('roast', being 'rot' around 'gas' missing the g) was valid!