Like many of us, actress Emily Watson can distinctly remember her whereabouts on July 7, 2005, when suicide bombers coordinated attacks across London, killing 52 people and injuring more than 700.

"I was at home in London, heavily pregnant with my first child, about to go and get the Tube," recalls the 48-year-old, who grew up in Islington and is known for her roles in Angela's Ashes, Gosford Park and, most recently, A Royal Night Out.

"We turned on the radio and heard there'd been an incident. I remember so much confusion and then the chilling moment when the bus explosion came. A 'power surge' became 'London was under attack' (at first, the explosions were thought to be down to a power fault, before the full horrifying picture emerged)."

Although Emily, who has two children with actor husband Jack Waters, wasn't "directly affected" by the attacks, she felt a "call of duty" to accept the role of a grieving mother in A Song For Jenny.

The standalone drama is based on the memoir by former vicar Julie Nicholson, whose 24-year-old daughter Jenny was killed in the bomb blast at Edgware Road tube station.

Adapted by the Irish playwright Frank McGuinness, who spent the best part of four years writing and caring for the script, it captures the twist of fate that saw Jenny take the Circle line that day and traces her mother's response, from the moment she hears about the attacks to the news that her daughter is missing, and then the confirmation that Jenny is among the dead.

"Obviously, there are many stories from that day. Each unique," remarks Watson. "Julie is a natural storyteller, and very significantly, a priest who lost her child to what purported to be, however twisted, a religiously motivated act. Her religious faith was very shaken and she's no longer a priest. And though in honouring her daughter she drove herself to the edge, staring into a dark abyss, ultimately she chose humanity, and chose love over hatred."

The actress met with the author, who stepped down from the pulpit in 2006 because she felt she could no longer preach forgiveness, shortly after reading her book.

"Julie shared very intimate, raw and tender memories with me. She's incredibly generous," the actress adds.

The bereaved mum admits it was tough to give the book over to strangers, but says "I felt privileged that someone else wanted to tell my story and Jenny's".

Producer Liz Trubridge, and her colleagues, met with Jenny's family, and assured them A Song For Jenny would be a collaborative process, not least while the script emerged.

"Frank talked to me regularly, and I saw each draft and was able to comment and have some input," reveals Nicholson, who was also included in the process of casting somebody to play her daughter.

"I said from the outset that it wouldn't be appropriate to search for a 'lookalike' Jenny, but that an actress should be found who could capture the essence of her, ideally someone who was not already a known face."

Nicola Wren, who was eventually chosen for the role, was straight out of drama school, "which I felt Jenny would have approved of", notes Julie.

As to how she felt having Watson, a two-time Oscar and four-time Bafta nominee, playing her, Nicholson says "blessed".

"I believed it was important that whoever played me should be a mother, and understand viscerally the powerful bond. Emily is an extraordinary and courageous actor who takes risks. Her stillness and quietness is as powerful as her actions and spoken words. There were times when I didn't know if I was watching Emily or myself."

Emily, who starred in the Fred and Rose West drama Appropriate Adult, is no stranger to emotionally-charged roles, but says this was one of her most challenging to date.

"Having to inhabit the deep traumatic grief and shock was pretty hard," she admits.

"To contemplate what happened to Julie's daughter happening to my own, is a brink that I have to pull back from. In rehearsals, it was as if we had to stop ourselves from letting everything flow because it was too much, and we had to hold it back and wait for the camera to be there because it was so devastating.

"Enjoy is not really the word - but there was a great sense of serious commitment among the cast and crew. Unlike anything I've experienced."

Nicholson has watched an early screening of the drama, and notes: "I felt like I had Jenny back again for a while, but then of course, I had to lose her all over again.

"The scenes with Jenny and her sister and brother were funny and joyous. It was very poignant watching my father, uncle and aunt, who are no longer here, and I felt the film captured all the relationships so well," she continues.

"I wept, of course, I think from beginning to end. I feel that something very beautiful has been created, a painfully bittersweet gift, but nonetheless, a gift."

A Song For Jenny is available now on BBC iPlayer