Script editor Rob Valentine said: "I’m a huge fan of the Sixth Doctor and Mrs Clarke. Colin Baker and Miranda Raison work beautifully together, and Constance brings out a very particular side of Ol’ Sixie, so the prospect of helping to bring a whole new set of their adventures into the world was extremely enticing. 

"It also meant I got to work with Julian Richards, Ian Potter and Paul Magrs, three writers I greatly admire, and their stories are terrific. There’s a particularly unsettling Dalek story, multiple Mrs Clarkes, and the return of a certain other traveller in space and time whom the Doctor was not expecting to meet again. All in all, you’re in for a cosmically good time.”

Writer Paul Magrs said: "The Key to Many Worlds was a great chance to reunite two great voices - Katy and Colin - from The Wormery all those years ago, and to twine them not only in one of my favourite Doctor Who genres, the historical saga, but also in one of my favourite science-fiction tropes, the mind-swap.

"With the casting of Tonio, Marco Polo’s boyfriend, I really lobbied for my late friend Antoni [Anthony Townsend] to get the part. Something I never, ever do, but this time I really wanted my friend to get what they'd always wanted – a good part, specially written for them, in a wonderful new Doctor Who story. It's three years since the writing and recording and Antoni is no longer here. I can't wait to hear their voice again."

Colin Baker added: "Upon reading this script, my first reaction was, oh, good, another one that's good and clever and touching and rather exciting. Things get turned on their head!"

And Katy Manning said: “It's been an absolutely joyous day and I actually climbed into the skin of Colin Baker! I've got grin-ache! We're so lucky, Paul Magrs is a wonderful writer."