The actress will return as Kate Stewart, the daughter of Brigadier Lethbridge-Stewart, in the special later this year. She first appeared in the role in 2012’s ‘The Power of Three’.

Redgrave told What’s On TV: “If I tell you anything at all I’ll have to kill you and then kill myself. I wasn’t even allowed to tell my children I was actually in it for about four or five weeks! Once we filmed in Trafalgar Square I knew it would be impossible to keep that under wraps! But what do you do? I was going to work and not being able to tell anyone what I was doing! It was like joining bloody MI6!”

The Frankie star then teased: “What I can tell you is that it’s absolutely bloody brilliant! I think it’s an astonishing script. Steven Moffat is an outrageously good writer and as always with Doctor Who, it’s philosophical, it’s deep, it’s an adventure story and if you’re a kid, you’re going to be taken on an extraordinary ride.”

She added: “It raises all sorts of interesting questions for anyone who wants to go deeper with it. It’s funny and then on a sixpence it breaks your heart. It’s just wonderful!”

> Catch up on all the 50th Anniversary Special news and rumours so far in our summary.

Are you looking forward to the 50th anniversary special? Let us know below…

> Buy Series 7 Part 2 on DVD on Amazon.

> Buy Series 7 Part 1 on DVD on Amazon.

Watch the Series 7 Part 2 trailer…