Sitting opposite the biggest name in British television, Steven Moffat, the man responsible for Doctor Who and Sherlock, I'm filled with trepidation.

To my relief, Moffat is perfectly amicable and my nerves are allayed as he offers up a few nuggets about the forthcoming series of the show.

“The Doctor is going to have a Western adventure,” he says. “We’ve been filming in Spain, which just looks amazing, and we’ve achieved some of the biggest production value ever. It’s going to include the most Daleks ever, too.’

Moffat reveals that series 7 will include several one-off episodes this time compared to the epic overarching story that spanned the last series, in order to give the show a different feel.

The new series will also see the departure of the Doctor’s companions Amy Pond and Rory Williams played by Karen Gillan and Arthur Darvill. “He’s going to lose his Ponds in episode five and that’s going to be huge and heart-breaking,” he says, promising a memorable, if unhappy, ending.

Is Moffat gutted that Gillan and Darvill are leaving? “I don’t mind. I think people leaving is fine, Doctor Who thrives on it in a way. But you don’t really know if you’ve enjoyed a story until it’s over. So I wanted to build a proper ending, I didn’t want to be caught in the hot as I have been occasionally in the past on other shows.”

Indeed Moffat is a seasoned television writer whose credits include the Bafta-winning Press Gang, a children’s drama series which focused on a school newspaper, and semi-autobiographical sitcom Coupling, based on Moffat’s relationship with his wife Sue Vertue. He also co-wrote the screenplay for last year’s Hollywood blockbuster The Adventures of Tintin alongside Edgar Wright and Joe Cornish.

Moffat took over from Russell T. Davies as the Doctor Who boss in 2009 but he had worked as a writer on the show since its 2005 revival. The first challenge he faced in his new role was to replace David Tennant’s immensely popular tenth Doctor and find a new companion.

“When we announced Matt [Smith], everyone was folding their arms and saying ‘What?’” He was a wild card, a relatively unknown actor, and yet Moffat managed to keep the series as popular as ever.

Former Emmerdale actress Jenna-Louise Coleman will be the Doctor’s new assistant. So far Moffat has refused to give any details about Coleman’s character but does she share any traits with Amy Pond?

“In one sense all the Doctor Who companions, for very, very good, dramatic reasons, have certain things in common. There are things that you need if you’re going to be the best friend to the Doctor and if you’re going to run away in a blue box. They’re always saying: ‘Why are they young?’ and I’m saying ‘Why do you think? Would a middle-aged person abscond in a magic caravan?’”

“The differences, I think, often really come from the actor. [Coleman] is a very different actress and it’s already feeling different.” Coleman will make her debut in this year’s Christmas special which is being filmed at the moment. Moffat explains that while he was writing the episode he listened to copious amounts of Christmas music to help him get into the festive spirit. He repeats to me the sage advice he was given by Davies before he left: “When it comes to hideous moments, Steven, when you’re writing Christmas in the middle of a heat wave, turn on the air conditioning and play the music.”

Along with the Christmas special, next year marks the 50 anniversary of the show. Could there be a new Doctor as part of the celebrations? The answer is a resounding no.

“The search hasn’t started. Matt Smith will be the Doctor forever.”

He keeps a tight lid on any details about the 50 anniversary despite the barrage of questions from fans asking which classic monsters will be resurrected. “One thing I’m slightly concerned about is that you get something like the 50, people start talking like it’s a full stop. I want to build for the future. I want to set up the next 50 years and to say: ‘Here are the new monsters we’re introducing.’ Rather than doing something which might seem like a farewell tour, because it certainly is not.”

Leaving Time Lords and the Tardis to one side, I ask Moffat about his other ratings winner Sherlock, a modern take on Sir Arthur Conan Doyle’s super sleuth. Along with co-writers Mark Gatiss and Stephen Thompson. They have now chosen the short stories that they will be focusing on in series three and last week Moffat gave fans a small teaser of what was to come by revealing three words: “Rat, wedding, bow”.

Sherlock mania has started spreading Stateside but how does he feel about Elementary, another modern retelling of Sherlock Holmes, set in America? Especially given the controversial decision to cast Lucy Liu as a female version of Holmes’ sidekick John Watson? “I can’t comment on their artistic decisions at all. That wouldn’t be fair or reasonable. All I’ve said was, CBS approached us about adapting our version […] and we said no because we’d had some bad experiences in the past. So they went, ‘well if you ever change your mind, get in touch’ and the next thing we heard they’d just decided to go and do one anyway. I think it’s remarkably bad behaviour.”

As a lifelong fan of the great detective, Moffat says that his favourite Sherlock Holmes’ story is The Speckled Band. “I think when I was a kid that was just about the most exciting thing a short story could do to you. That doesn’t necessarily mean we’ll do it in the show.” Any other stories he’d like to adapt in future? “There are quite a few, but mostly I’m looking for elements because those stories are all very, very short and the big one, The Hound of the Baskervilles has been done so many times.”

“We have to think of ways to take one element and combine it with this element or that character in that story. You have to take a magpie approach to it because otherwise you wouldn’t have a long enough story.’