Last week saw the surprise news that John Simm’s Master would be returning to BBC sci-fi series Doctor Who, with the alien villain appearing alongside his regenerated incarnation Missy (Michelle Gomez) in a story for the upcoming tenth series.

And now showrunner Steven Moffat has exclusively revealed to RadioTimes.com the reasoning behind the decision to bring Simm back, which was apparently more than seven years in the making.

“Well I’d always thought when we got Michelle that at some point it’d be fun to have a story where two versions of The Master meet,” Moffat told us at the BFI & Radio Times festival this weekend. “I’m sure everyone’s thought that at some point.

“But in a way you can’t do that straight away. You’ve got to let the new one become the definitive iteration as it were, and then you can bring in the wonderful John Simm.”

And it seems the germ of the idea had even existed a few years before Gomez’s casting, with Simm privately telling Moffat he’d be happy to return during a screening for his last appearance in the sci-fi series.

“He’d always said that he’d quite like to come back,” Moffat recalled. “When I spoke to him at the End of Time screening… I’m really grassing him up at the moment, it’s terrible.

“And at the End of Time screening he said ‘Well I feel that now David’s steeping down as the Doctor I feel I should step down as The Master. And he came and found me at the drinks and said ‘I didn’t mean it, I’m perfectly happy to come back.’

“So we went obviously the route with Michelle, and once Michelle was established and people were loving her we thought it was time to get that tremendous meeting. John was so keen to come back.”

And it seems the decision has paid off, with Moffat praising the two Masters’ chemistry onscreen in the upcoming series.

“They are absolutely wonderful together,” he said. “They are hilarious, it’s absolutely gorgeous. You’re in for a real treat. A real treat.”

Sounds like a regeneration reunion well worth the seven-year wait.

You can watch the full Steven Moffat interview above

Doctor Who returns to BBC1 on Saturday 15th April at 7:20pm