Earlier in the day he had failed to get beyond the wall of tourists watching the changing of the guard at Buckingham Palace, in order to present his credentials. But that did not deter him from trying his luck with Tony Blair.
"The south-west wind blows 24 hours a day over Texas," he drawled, unfurling his blue and yellow flag. "It figures that we are fairly resilient characters."
His persistence did indeed pay off. Well, at least he made it as far as the gates at Downing Street. "Hello, how are you? I'd like to present a letter for the Prime Minister."
His jaunty manner did little to impress the attendant policeman, whose polished appearance contrasted with Texan visitor's weathered look.
Realising that he was no more going to hand-deliver his documents to Number 10 than have tea with the Queen, the would-be diplomat settled for sealing the envelope. It was addressed: "For the personal attention of the Right Honorable Prime Minister" and stamped with a Lone Star State silver sticker.
Perhaps Mr Blair would look upon him sympathetically. Not only was he a new prime minister and a "fresh outlook", but: "Tony Blair, Bill Clinton and Robert Hertner all played in different pop bands together." Sorry? "In different bands in the same time frame," he said.
In one letter, Mr Hertner invoked "the protocols under the Doctrine of Retroactive Recognition in accordance with the Law of Nations in respect to the treaty between Great Britain and the Republic of Texas circa 1840."
He requested copies of the treaties and documents with a view to "normalising relations between our two great nations."
He handed over the package and reflected: "They've been officially delivered, in public, in front of witnesses, and that's the best I can do at the moment."
But that left the small matter of Her Majesty. "I'm not exactly sure how to contact the Queen with my documents," he puzzled. "I'll fax them? They fax me? I don't know."
Mr Hertner, who was dressed in his father's battered bandero, a pair of cracked, leather cattlemen's boots, and Navajo jewellery, claims to have "tens of thousands" supporters back home. He summed up his message: "Anyone can become an American, but you have to be born a Texan."
Three weeks ago, he turned his home in Kensal Green, north-west London, into the "Embassy of the Republic of Texas for the Sovereign Nation of Great Britain". "Much like Tony Blair, I've been a diplomat for exactly three weeks," he said, reaching for another point of contact.
Mr Hertner wound up his flag and headed for home, saying: "I think we're going to do the intelligent thing and go get some nachos for brunch."