Ruth Jones, co-writer and star of BBC comedy Gavin And Stacey, takes on the role of the larger-than-life Carry On actress Hattie Jacques in a one-off film for BBC4. The film, which reveals her secret sexual liaison with her handsome young driver, is based on the book by Andy Merriman – Hattie Jacques, An Authorised Biography.

The film tells of Hattie’s secret affair with a younger man, while married to Dad’s Army star John Le Mesurier in the early Sixties – a hotblooded romance in stark contrast to the matronly roles that made her famous.

Hattie Jacques began her career in the Forties, first gaining attention through radio appearances with Tommy Handley on It’s That Man Again and later with Tony Hancock on Hancock’s Half Hour. From 1958 to 1974 she appeared in 14 Carry On films, often playing the matron. She had a long professional partnership with Eric Sykes, with whom she co-starred in his long-running TV series, Sykes.

Jacques was married to John Le Mesurier from 1949 until their divorce in 1965. She died in 1980.

The cast of BBC4’s Hattie also includes Robert Bathurst as John Le Mesurier, Being Human star Aidan Turner as John Schofield and Jeany Spark as Joan Malin.

How did you feel when asked to play Hattie Jacques?

SHE’S a comedy heroine of mine and I’m thrilled to have been asked to play her. She was an incredibly talented and fascinating woman both on and off screen and so much more than just the ‘funny fat lady’. The Carry On films are a British institution, they’re so funny – I was very privileged to be playing Hattie Jacques, she was such a much-loved actress.

How different was she to the woman people know through the Carry On films?

There was more to Hattie Jacques than the public persona of those films. I was surprised.

Because most people only know her for playing the austere matron in the Carry On films, her private life in this film will surprise, but I feel people will enjoy discovering this other side to her.

What’s your take on Hattie Jacques’s love triangle?

My understanding is that Hattie loved her husband, John Le Mesurier, very much and it was a wonderfully warm and close relationship. But John Schofield offered a raw passion and romantic nature that Le Mesurier could not compete with – and that Hattie never quite expected in her life. However, Hattie could never stop loving the man she married and that’s what makes the drama so interesting.

What drew you to the project?

Hattie famously said in an interview “If you’re fat, you’re funny,” so I really wanted to see the person behind the funny fat persona. Sometimes people don’t look beyond the character and I can understand that feeling and where Hattie was coming from.

Do those stereotypes still exist today?

I don’t think we have quite the same over-simplified view of the world. Fat Friends explored similar subject matter. Nessa, from Gavin And Stacey, has fun with her size and is comfortable with it. I feel lucky that there are more roles for larger actors. I don’t think I’ve experienced the same stereotyping Hattie Jacques seemed to face in her acting roles.

Did you feel the pressure of playing a leading lady?

I love working in a team, in an ensemble. I don’t know if I’m wholly comfortable always being in the spotlight. I was thinking, ‘oh God, it’s Ruth Jones throughout the whole of the drama. What if people don’t like me?’ But I knew that through the weeks of filming, I just had to be so focused on the job – and I was.”

How were the costumes?

Hattie’s style was very much Fifties hourglass.

Down to the vintage remake of a girdle which the costume department made for me, they were very much the Spanx of their day – they just hold everything in and it all added to the joy of wearing the clothes and playing the role. The costumes made me feel really feminine. They made you feel part of that world and not modern-day.”

How did filming Hattie rate with other acting roles you’ve done?

It’s definitely one of the best jobs I’ve done and I really wanted to do her justice. I think from the script to screen, everything has been done with the greatest of respect to her.

Any final thoughts on Hattie?

I think she loved performing and she loved the public. She also did a lot of charity work. Something I read about, which I loved, is she would sell her autographs, or only give them out, on the condition people would donate to a charity.

I felt a real affection for Hattie. I’m so fortunate as an actress, I’m always dressing up and playing make-believe.

With this role, I was able to get a glimpse of what it must have been like to be Hattie Jacques and, when filming was all over, I knew I was going to really miss her.”

■ Hattie is on BBC4 on Wednesday at 9pm