Having a law on the books is great, but I wonder whether withdrawing the notes from circulation would be more effective by making it a pain to do large scale off the books transactions.

With the ban on cash transactions that you are talking about, it just raises stakes of getting caught.  But if the largest note was a 20 or 50, then it would be a whole lot more difficult to do these things off the books.  Unless you fancy carrying around a trunkload of the stuff. Presumably doing a transaction all in 20s would mean it would weigh 25 times as much as in 500s.

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg