Producer David Richardson said: "Can it really be fifty years since the Fourth Doctor arrived in Doctor Who, in the wonderful Season 12, supported by the glorious Sarah and Harry? That really is something to celebrate, and I felt the best way to do it would be to return to 1975 and add an extra story onto the run.

"So The Curse of Time follows on from Revenge of the Cybermen and comes before Terror of the Zygons, as the TARDIS is diverted on its way to Scotland. Jonathan Morris has written a smashing tale that fits perfectly into both the Nerva Beacon arc and the Hinchcliffe years. Happy anniversary to the Fourth Doctor!"

Tom Baker, reflecting on being cast as the Doctor half a century ago, said: "It changed my whole life. And it also marked a time of pleasure and laughter and excitement and everything like that, which has lasted right the way through. When I came to record this special anniversary story, I feel just as enthusiastic as I did all that time ago. It just comes back to me and it makes me so happy. When I start learning these lines, I become the Doctor."

Sadie Miller, who inherited the role of Sarah Jane from her late mother Elisabeth Sladen, added: "From that first episode where he's trying on all the different costumes, you can really see Tom as the actor trying on the different ways that he wants to be the Doctor, and then deciding in a very Tom Baker way that actually he's going to be all of them, all the Doctors, all the time.

"But he's so clever that he has that spontaneity to it as well, and the child in you instantly connects with that energy. That’s why Tom has such a wide-ranging appeal."

Christopher Naylor, playing Harry Sullivan, said: "I've got wonderful, happy memories of watching Tom, as a boy all those years ago. I do remember Ian Marter as Harry, and feeling very sad when Sarah left the show. With due respect to all of the other actors who've played companions since, I don't think that trio was ever equalled.

"The Curse of Time has a wonderful historical feel, as it rather cleverly takes us into the Middle Ages, although of course, it's actually set thousands of years in the future. And it has a real feel of those '70s stories where they were wonderfully atmospheric and exciting in how the sense of place is very well captured, and there's a real sense of jeopardy."

Scarlett Courtney, Angus Dunican, Terence Wilton, Helen Goldwyn, Sadie Miller

Andrew French, Helen Golwyn, Scarlett Courtney, Angus Dunican, Christopher Naylor, Sadie Miller, Rosemary Ashe, George Naylor

Christopher Naylor and Sadie Miller

Christopher Naylor, Tom Baker, Sadie Miller © Paul Midcalf