Smith is a hugely likeable mix of chirpiness and emotional-car-wreckage.She laughs a lot, but there are also numerous points in our conversation when she fans her beautifully manicured fingers in front of her blue-grey eyes in an “Oh no, I’m welling up” gesture.I felt I was saying goodbye to a truly incredible and inspirational lady.” Think what you will about Ronnie Biggs – Great Train Robber, prison escapee, and one-time Rio-residing fugitive – but Smith is about to challenge prejudices that even she admits she had before playing his former wife.

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“It was brilliant.” The second was for her supporting role as Doris in the revival of the Terence Rattigan play, Flare Path.

“I was filming Mrs Biggs in Australia so I sent my mum and dad along to represent me at the awards ceremony.

“For goodness sake,” she laughs, combining the two sides of her personality perfectly. If I carry on like this, my false eyelashes are going to come off!

” The source of much of her sorrow lies in the elements of Charmian’s story that echo her own.

It felt so right because, basically, I owe them everything.” Raised in Epworth, a village near Doncaster, she and her brother, Damian, who is lead singer of the band, The Torn, got the performance gene from their parents who are – even now – a country and western singing duo, The Daltons.

Her upbringing, she says, was firmly working class.Even when she had to finally let Ronnie go – when he had settled in Rio and was having a child with another woman [which, thanks to Brazilian law at the time, meant as a parent of a Brazilian he could never be extradited] – she still didn’t fall apart.She went back to university and made the best of herself for the sake of her children.” When I meet the 31-year-old actress – at the South Bank offices of ITV – her Charmian-red hair has been bleached back to its usual blonde, but the residual feelings remain.“Charmian invited me to her home in Australia and showed me the letter Ronnie sent back.It was totally heartbreaking and showed a really sensitive and intelligent side of Ronnie that the public would probably never guess he had.When Sheridan Smith left Australia at the end of filming the controversial new drama, Mrs Biggs, there were emotional scenes between herself and the real-life character she plays to match any that viewers are about to see on screen.