Hi Murky, I looked with interest at #42 and found it was my post! Not sure which bit was confusing but I should probably have said 'the down clue letters started to look like a phrase I know, which directed me to the theme and therefore the key to the multi-letter squares. Then, by process of elimination, I knew what I was looking for for the final element which must be part of 1a'.
Solving the parsing of a couple of across clues, or identifying the misprints in them, again was helped by process of elimination, thanks to the 'Books recur....' part of the preamble.
I may have been a little opaque as a result of not wanting to give anything away regarding the theme.