Writer, Chris Chibnall, has written 5 episodes for Doctor Who in the past with his most recent episode being Series 7 episode: The Power Of Three.
Speaking to Cult Box, the Broadchurch writer said:
"I hope to [return for series eight], and Steven [Moffat's] been good enough to ask. So we'll just see if all the schedules figure out and stuff like that. But it's always lovely to be on the list."
"It's a delight to write a Doctor Who (episode). Steven is a great show runner to work for, and you always do a Doctor Who if there's time, because you can tell stories you can't tell anywhere else."
[Source: Digital Spy]