You’ve written this year’s Christmas special. What can you tell us about it?
Obviously, we’ve just lost Clara so I didn’t want to go straight into a new companion, and it’s not right to set that up at the end of a series where you’re all exhausted. So, I’ll be honest, I brought River Song back in because I thought there’s a possibility I’d never write it [Doctor Who] again so that’ll be my goodbye. But also I really fancied it. I hadn’t written River for a couple of years and I’d always loved writing for her and I’d missed her. I thought, given that I’m knackered at the end of a series, what would be a treat for me would be getting Alex Kingston back onto Doctor Who. 

And luckily she was available.
It was. She’s very busy and successful. But we spoke to her quite a long time in advance and I sat in the office one day and said, “Can I have River? Bring River back?” Our current series of Doctor Who gets quite dark towards the end so I thought it was time at Christmas to take it back up. 

So there’s a deliberate change of tone?
It’s towards the comedy thriller end of the Doctor Who market. It’s light, and quite emotional at the end. The big deal is Doctor No 12 encountering River and vice versa. It’s strange because they’re now, in our human terms, the perfect couple. They’re both sexy older people. It’s always been slightly strange before with Matt [Smith]. Now, yeah, they could actually be married. It works. She’s great with Peter [Capaldi]. She genuinely hasn’t aged a day since she did The Silence in the Library [her debut story], which we made in 2007. She looks amazing. She totally gets how to do that part. And she’s very, very funny.

It’s a great romp for Christmas day but – and you’ll be pleased – there’s not a lot of Christmas in it. It starts with a big bit of Christmas at the beginning and then we ignore it and go off on a chase, really, with Mr & Mrs Who. One thing worth saying about it is, if you’re a long-term fan of the show, you might be interested because there’s been a lot of talk about the Doctor and River’s glory years but we’ve never seen them. Here at last we see them as Mr & Mrs Who at the height of their powers. It’s great fun.

Where is it in River’s timeline?
It doesn’t really matter but for those who care, it’s immediately after The Angels Take Manhattan [2012] so she’s just seen Matt Smith’s Doctor lose Amy and Rory, and obviously before The Name of the Doctor [2013] because she’s dead in that.