mct16:
Michael Farr, author of "Tintin the Complete Companion", has suggested that Müller was based on Georg Bell who worked with the Nazis and was allegedly involved in an attempt to flood Soviet Russia with fake Roubles.
There's not much written about this Georg Bell character on the 'net, but what I could find out is very intriguing. Michael Farr says he was "a Scot living in Germany who took German nationality", but the German Wiki page on him says he was born in Germany. Elsewhere I found mention of him having Scottish ancestry. He'd apparently worked as a German-British double agent, in Germany as a spy for the S.A. (Hitler's private army before the Nazis seized power in Germany). Bell was sent to London in 1931 with plans to build a capitalist pact between England and Germany against the U.S.S.R. (the fake Roubles plot, no doubt) and he was also involved in an S.A. plot to topple Hitler. The Nazis evidently thought him far too dangerous and he was executed in 1933.
I also found on the web a report from
The Daily Express, April 5th, 1932, which says that on one occasion Bell - who'd been employed "on work of international importance by the British Secret Service" - was refused permission to enter Britain via Harwich, and he'd panicked, fearing that he had as many enemies as "friends", many of whom wanted him dead.
Bell seems to have been an extremely shady figure, an opportunist with no political convictions or scruples. That seems to fit well with the character of Müller who, rather than a Nazi sent to subvert the British economy, worked mainly for personal gain (helping anyone else who could help him in the process). The Nazis did make plans to flood the British economy with fake cash in Operation Bernhard in 1942, but Britain and Germany weren't at war at the time
The Black Island was written (1937-38), so it seems unlikely that the Nazis would've attempted it at this point.
Note: The German Wikipedia page says he had Scottish "roots" through his father, but doesn't say that his father was himself Scottish, or whether either held British citizenship.