Every Saturday, he would go to his local branch of Barclay's Bank to get enough money to see him and his family through the week.
But this time he had no such luck. He arrived at 12.31pm, a minute after closing time, and the doors were firmly shut.
Looking back on that day with the benefit of hindsight, though, he is probably glad he was late.
Because had he been on time, he would have never come up with the invention which transformed the world of banking – the "hole in the wall" cashpoint.
It is 50 years ago today since the first automated teller machine, or ATM, was unveiled at Barclay's Bank in Enfield, north London.
Comedy actor Reg Varney – who would go on to star as Stan in On The Buses – became the unlikely trailblazer for the technological revolution when he became the first person in the world to use a cash dispenser.
Shepherd-Brown, who died seven years ago, later recalled how it was while lying in his bath after his wasted trip to the bank, that he hit upon the idea of a 24-hour cash dispenser.
As he seethed at the inconvenience of the experience, and how he had ended up going to his local garage to cash a cheque, that he though there must be some kind of machine that would allow people to get their money when the branch was closed.
"I thought of the chocolate vending machine where money was put in a lot and a bar dispatched – surely money could be dispensed in the same way," he said.
Later that year, by chance, he bumped into the chief general manager of Barclays Bank who was about to have lunch. Over a pink gin, Shepherd-Barron asked him for 90 seconds to pitch his idea for a cash machine.
"I told him I had an idea that if you put your standard Barclays cheque through a slot in the side of the bank, it will deliver standard amounts of money around the clock," he said.
"He said, 'Come and see me on Monday morning'."
Barclays commissioned the computer whizzkid to build six cash dispensers, the first of which was installed at Enfield.
Remembering his National Service number, the original plan was that customers would identify themselves with a six-digit number. However, his wife Caroline thought this would be too long.
"Over the kitchen table, she said she could only remember four figures, so because of her, four figures became the world standard," he said.
In the early days, before the arrival of plastic bank cards, customers would use hole-punched vouchers allowing withdrawals of £10 a time, which had to be obtained in advance and inserted in the dispenser when needed.
Despite a few problems with vandalism, the idea quickly caught on. Today there are 70,000 cash machines dotted around the UK alone, which were used to withdraw a total of nearly £180 billion last year.
Shepherd-Barron was awarded the OBE for his services to banking in 2005, although many will wonder why he was not honoured earlier.
To mark the half century, Barclays has coloured its cash machine at the Enfield branch gold.
And despite the rise in other new technologies such as online and mobile banking, the ATM remains popular 50 years on.
The UK record for the most cash withdrawn in one day was broken as recently as December last year as Christmas shoppers withdrew £730 million.
Trade association Payments UK recently predicted that the rapid growth in the use of contactless cards means cash will be overtaken as Britain's most frequently-used payment method by the end of next year.
But Payments UK has said its forecast did not herald the demise of cash, as even in 10 years, cash is still expected to make up more than a fifth of all payments.
Raheel Ahmed, head of customer experience and channels at Barclays, says: "Even though recent years have seen a huge uptake of digital banking and card payments, cash remains a crucial part of most people's day-to-day lives, whether it's paying for groceries or doing the office coffee run.
"We're very proud of the role that Barclays has played in the history of the cash machine."
This month also marks 30 years since Barclays introduced the debit card to the UK.
Jeremy Light, managing director of payments at technology services company Accenture says the machines of today are changing with the times to keep up with customers' needs.
He says: "The ATM is changing, as it takes on a new role to complement online banking. Donating to charity, buying stamps or even applying for a credit card are all possible and may come to your local ATM.
"Smarter technology means ATMs are more secure and versatile today, for example cash withdrawals using a mobile phone instead of a card. ATMs perform an important role in the UK economy and maintain customer interactions with a bank. Perhaps cash will always be king."