I also don't think I'd heard the phrase 20d 23d before, but 23d jumped out, the meaning of the phrase became clear and eventually so did the first word.
Having grown up in London, I am familiar with 10a but others might have come across it as a description of a piece of clothing in a Lonnie Donegan song. 19d 25a is a phrase I often use, I'm determined to explore the road in Paris sometime.
Like the violins and parasites!