Alex Kingston said: “We have gone post-Library! We’ve done another episode in the past where I was within the database, which I loved, and I thought that’s the only way you could go forward with River, given her situation. So, I was actually really surprised when this box set came my way, and that we are now so far advanced in the history of Earth that she is able to be brought out as data and put into a cloned body. So very, very clever!

“I love this series of episodes, it’s very dark. And although it's science fiction, it feels very close to what our real future as a planet could be heading towards – not only with the solar flares that are out of our control, but with the billionaires who are in many ways responsible, and who will try and save themselves at the expense of everybody else. It’s really timely that Rob has written this story.

“I was thrilled when I heard that Garrison Clay was going to be played by Greg Wise. I can't believe that he has never done anything on Doctor Who before! It’s lovely to see him coming into the recording studio and being so excited, and it’s such a wonderful family for him to join.”

Her co-star Greg Wise added: “This is the first time Alex and I have worked together, so it’s been lots of fun. Pretty much my entire career has been playing baddies, but it has to be said baddies always have the best lines. And I love the interaction of my character with River – she’s glorious!”

Producer David Richardson said: “Robert Valentine has done something remarkable with this four-hour drama, which tells the story of the last days of planet Earth before it is destroyed by solar flares, a story told from the perspective of a woman temporarily brought back from the dead. It’s heart-breaking and exciting and doom-laden and, perhaps when you least expect it, uplifting and inspiring.

“When every single day during the recording, your leading actress says “This script is brilliant”, you know you’re onto a good thing. Even better, at the end of the last day, Alex popped the script in her bag and announced that she loved Last Words so much she was taking it home to keep and treasure.”

Writer Robert Valentine added: “Originally, I thought I was going to be writing the final ever River Song story. Quickly I realised that I wasn’t, but I thought it would be fun to write something that feels like it’s the last we’ll ever see of River, and actually give that problem to her. Her life is usually intertwined with the Doctor’s, so I thought, what is her life like after her great relationship is over?

“And, the brief was to do a prequel to The Ark in Space, which is a Doctor Who classic, at the same time. When I went back and rewatched it, I realised that I wouldn't be allowed on this ship, I would have been one of the ones left behind. So I thought, let's give River a mission that leads into The Ark in Space, but with sympathy for those people who aren't on the Ark.”

Alex Kingston in the Soundhouse recording studio

Alex Kingston, Andrew James Spooner, Issy Van Randwyck

Alex Kingston, Jacob Daniels, Shogo Miyakita, Robert Valentine and Greg Wise