Dangerous Assignment - Prevent Disclosure of Compromised Codes

Agent Steve Mitchell walks into the Commissioner's office and says "Morning Commissioner, Ruth said you had an assignment for me". The Commissioner says "I do, Steve. Suppose you had a secret code, and you suddenly found out that a hostile country had succeeded in deciphering it. What would you do?" Steve says "Well, I'd quit using that particular code". Commissioner asks "Would you?" Steve says "Oh...wait a minute. If I knew that they'd broken the code, but they didn't know I knew, I think I'd keep on using it". Commissioner asks "Why?" Steve says "Well, that way I could send a lot of phony information that would probably tie up their strategy". Commissioner says "That's absolutely right, Steve and, that's just what's been going on for the last six months. We found out from a very secret source that a certain country had broken one of our diplomatic codes. One we've been using with the French government". Steve says "And we've been using that same code since, sending false information?" Commissioner says "Yes, we've managed to mislead that particular country on several vital issues during that period". Steve says "Well, it sounds like a pretty neat little racket". Commissioner says "It is indeed and we'd like to keep it going as long as possible but, obviously, the success of it depends on their not knowing that we know they've broken our code. And that's why the whole thing is hanging in the balance right now". Steve says "You mean they found out?" Commissioner says "Not yet, but unless we can prevent it, they're going to find out very soon". Steve asks "How do you mean?" Commissioner says "Somewhere along the line here in the States has been a leak. Steve, somebody found out our little secret and right now is on route to a meeting with a representative from that other country to sell them that secret". Steve says "Oh great, who is this guy and where is he now?" Commissioner says "We think that he or she is in London at the moment. Steve, we haven't the slightest idea who this person is, or where the meeting is taking place". Steve asks "Do we have any agents there?" Commissioner says "Yes, Slater's there at the Regency Hotel. Get over there and work with him. We must find out who this person is, and prevent this meeting from taking place".