Lord Haw Haw and the Officers Mess Clock
There were lots of rumours of this kind during the war and there is a whole file about them at the National Archives.The BBC carefully monitored the Germans' broadcasts to Britain and elswhere but did not find even a single example of such a reference.
I think these rumours must be put down, in most cases, to the agitated emotional state many people were in during the war and, perhaps, in a few cases to deliberate scares spread by Fifth Columnists.
There is also another possibility. Another file in the National Archives reports that MI5 were investigating whether malicious people in this country with access to radio transmitters were broadcasting these messages themselves. However, no real evidence of this was ever discovered.