Producer Dominic G Martin said: "The Legacy of Time was a wonderful celebration of Big Finish Doctor Who, but there was a question that we wanted to answer - just what were the First and Second Doctors doing on the lead-up to their appearance in that special?

"These two adventures answer that, featuring Stephen Noonan and Michael Troughton's wonderful portrayals of these Doctors and told, in natural Companion Chronicles style, from the perspective of companions who you wouldn't necessarily expect to see them with.

"Barbara Hambly has given us a wonderful story pitting the Brigadier and the First Doctor against the Master and his pet Kraken, and Simon Guerrier has written a captivating, mind-bending tale as the Second Doctor teams up with Harry Sullivan and Naomi Cross. These extraordinary little tales fit perfectly as a bonus addition to The Legacy of Time, and it's a thrill to share them with you."

Writer Barbara Hambly, an acclaimed science fiction novelist who makes her Doctor Who debut with this set, added: "I always loved the UNIT stories. It was almost like a separate series for Pertwee's first two years: a stranded time-and-space traveller stuck on Earth working for the everything-in-triplicate military. And yet, the spit-and-polish Brigadier was never a caricature - none of the UNIT gang were.

"Since first I saw Doctor Who back in 1982, I wanted to write a Who story, and when they asked me which companion I wanted to do, a UNIT story was my choice. Just like the 1970... but with better special effects!"

In The Kraken of Hagwell, Jon Culshaw narrates the story as the Brigadier later in his life, while also providing the voices of both the Master and a younger version of the Brigadier. Culshaw said: "I really enjoyed the format of this script - the audiobook sense of the narration, and how it then goes to the clips of the action where me and Genevieve and Stephen have been acting together, that hybridisation of narration and full-cast scenes.

"I’m playing the Master and the Brigadier, on the same page a lot of the time! Thankfully, with the Brigadier and the iconic Roger Delgado Master, the two voices are so distinctive and very different that one little pause, just a little gear change will get you there."

On reprising the role of Harry Sullivan for The Heartless Sea, Christopher Naylor said: "I really love this script. It's very claustrophobic and intense, very tightly focused on those characters. It's a real thriller, this one.

"I love working with Ellie Crooks, we have a lovely rapport, and I just think Michael Troughton is brilliant. He captures his father's performance so well, and it's so well written that people will really get a very strong Second Doctor vibe from this."

Watch Tom Baker in a prequel to Doctor Who: The Legacy of Time!

Stephen Noonan and Jon Culshaw with Nicholas Briggs

Eleanor Crooks and Christopher Naylor with Nicholas Briggs and Dominic G Martin