The TV programme, which arranges for wealthy individuals to volunteer at charities under an assumed identity, sent entrepreneur Ivan Massow to its base in Ferry Road Drive.
Staff admitted that they wondered whether the volunteer – who arrived under the name of Evan – might in fact be a Secret Millionaire.
But the cover story offered by Mr Massow and the TV crew that accompanied him was so convincing that they were still astonished when he revealed his true identity.
The charity’s managing director, Keith Robertson, said: “The production company contacted us at first and said they were making a programme about how the recession had affected lives and about people changing careers.
“They said that he was between careers and was from the insurance industry, which he is – but quite a large player, I now know.
“They said he was somebody moving from financial services to more caring professions.”
On arriving at the charity in May, Mr Massow, who made his fortune in the insurance and financial services industries, was shown around by staff.
He soon got his hands dirty, helping on the volunteer-run production line which each year puts together 9000 starter packs of household goods for people moving into their first home.
Mr Robertson said: “We also work in people’s houses alongside them and the aim of that is that we improve their living environment by decorating with them, but it’s also a form of support that we’re there with them. He worked in someone’s house all day, painting.”
The programme’s producers provided very basic accommodation for Mr Massow to stay in during filming, Mr Robertson said: “He was quite shocked by the circumstances of the housing situation they put him in. From what he told us, it was a fairly basic place, so I think he was put in a situation that he would be in if he was starting off without anything. I think he was genuinely quite shocked by the situation people find themselves in.”
Mr Robertson admitted he had suspicions that the new volunteer might a Secret Millionaire, but eventually abandoned them.
He said: “It’s an interesting journey, because at the start it’s a cover story that you buy into and then you think ‘Wait a minute, is this Secret Millionaire?’ but they’re so consistent with their cover story, even the production company, so you think ‘No it’s not. Yes it is. No it’s not’. So the whole thing about the reveal is very genuine, because you don’t know.”
Mr Robertson said staff and volunteers were delighted when Mr Massow revealed his true identity – and offered them a hefty cheque to help their work.
“It’s a very generous donation he’s given. I’m not allowed to say how much but it’s fantastic. Obviously, with the current economic situation demand for our services is high, and it’s more and more challenging to get funding. It’s probably the single biggest donation we’ve had from an individual.”
The programme will be screened on Channel 4 at 9pm on October 23. To find out more about the charity see www.freshstartweb.org.uk.