"The Holy Terror" (aka "The Saint vs. Scotland Yard) is one my favourites of the books of (usually three) Saint "novelettes", the other being The Misfortunes of Mr. Teal.
In "The Inland Revenue", Simon has to deal with equally implacable adversaries: a blackmailer known only as the Scorpion, and His Majesty's Inspector of Taxes ("Not that there's a great deal of difference. The same threatening letters, the same merciless bleeding of the honest toiler...").
"The Million Pound Day" — which would have been around 1931, of course — would cost at least a hundred million now, a sum certainly enough to prompt the associated shenanigans, in which the Saint rescues a man from torture and finds himself involved in a plot to swindle the Bank of Italy.
And "The Melancholy Journey of Mr. Teal" finds the Saint for once unable to talk his way out of trouble, and on the brink of being arrested — leading gracefully into the next book, one of the best of all Saint adventures, a romp through a Germany that was not yet quite Hitler's.
Incidentally, those who have been clubbed to the ground by O-Level French (or whatever the equivalent is nowadays) may doubt that anyone — in addition to being able to box, throw knives and play the banjo — could really speak as many languages as the Saint does. But there are such people: Sir Richard Burton (the explorer, not the actor) apparently absorbed languages like blotting paper, being able to pass the brutal Civil Service translators' exams after six months. Charteris himself was a polyglot, later writing a textbook on Spanish (which I would love to get hold of).
Pico della Mirandola (1463-1494) reportedly spoke eighteen languages; but as to whether he could play the banjo, we are not informed.
P.S. For a list of — and discussion of — all Charteris's Saint books, see my So You'd Like To... Guide