Karl Heinrich Meier
Karl Heinrich Meier was a Dutchman of German origin (then aged 24), who, with his accomplice Jose Waldberg, a German national (then aged 25), was hanged at Pentonville Prison in London on 10 December 1940, following their conviction at the Old Bailey the previous month. These were the first executions under the Treason Act.
They both spoke good English, and carried papers showing them to be Dutch refugees from the Nazis.
From the Daily Mail Weekend Supplement, 26 November 2011 - page 14:-
Mabel Cole was landlady at The Rising Sun in Lydd, Kent. The pub had been in the family for 140 years, so she was surprised when a stranger walked in at 9am one day in October 1940. Surely everyone knew the law prevented her from serving drinks so early?
Her surprise turned to suspicion when the customer, a man with a faint American accent, returned to ask for a brand of `cider champagne' that, though advertised on the front of the pub, had long been discontinued. `But the real giveaway,' recalls her nephew, Eddie Baker, 73, `was when he went to pay. Aunt Mabel told him, "That'll be one and a tanner" – a shilling and sixpence. She totally confused him with that bit of slang. He pulled some cash out of his pocket and held it out for her to take.' Convinced he was up to no good, Mabel had the police called – and German spy Karl Heinrich Meier, 24, was apprehended. He was hanged a few weeks later at Pentonville.
The Germans Are Coming, 6 December 2011 at 9pm, on the Yesterday channel.