Most people accept that Sherlock was playing Moriarty all along and that when Moriarty says "there is no code, doofus" that was no great surprise to Sherlock. He just pretended to act shocked etc...

So, the question is, at which point did he realise/work out that there was no code? Did he know all along, from the beginning, that something like this would be impossible, or was there a point where he did believe it?

There's that moment where they jump in front of the bus, grab the gun off the assassin and ask him what he wants. He says, "the computer key code, he planted it at your flat.." etc. And Sherlock's response is, "of course, the key code, he's selling it to the highest bidder..." Is he completely just blagging at this point, pretending for the sake of John and the soon to be dead assassin? Or does he genuinely believe that there is a code and he only realises there isn't later on, perhaps whilst hiding out at Barts?