Where was Gunpowder filmed?

“I've used it for Peaky Blinders, I made [a set of] Chinatown downstairs,” veteran set designer Grant Montgomery said when RadioTimes.com ventured up to the muddy northern set.

“I think it's rather funny to get all these Hollywood stars down to Dalton. Bill Nighy, now Kit [Harington] and Liv [Tyler] and Peter [Mullan] and Cillian [Murphy, star of Peaky Blinders] and the rest of them.”

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And the sheer size of the Mill allowed for plenty of interpretation.

“We utilised the whole thing to build other sets,” Montgomery said. “Inside you'll find the Tower of London, and the torture chamber. So that was built inside here.

“It's all fake, but [the doors] are based on an actual door in Newgate prison, the design of it. So it's inside this Mill and we just built all the walls and the cells and the torture chamber, where [Guy] Fawkes is tortured and his nails are pulled out, and all sorts of nice things.”

Also included inside the Mill itself are slightly more pleasant sets including the houses where priest Father Garnet (Peter Mullan) hides out with Anne Vaux (Liv Tyler), which are decorated to contemporary mores right down to the finest detail.

“There's a lot of painted panels,” Montgomery explained. “You think of all the panel rooms you go to and you see them all wooden and black, and actually they were painted. And if you were rich, you really painted your interior.

“It wasn't just plain brown veneers, it was a lot more colourful in that period. It's like some of the churches that have now disappeared, they were hugely colourful, amazing. I've been to York university and seen the recreation of the Queen's chamber, it was incredible. And it's all gone.”

He added: “The secret Catholic symbol was a strawberry plant – because when it was cut in half, it was the heart of Christ. So that's also [painted] here, so loads of kind of symbols that if you know about you'll get, but if you don't they’re just there.

“I used a lot of paintings that were references to El Greco, because his Catholic imagery is much more tortured, compared to the softer Italian/renaissance painting.”

The streets of 17^th-century London themselves, meanwhile, were built in an area next to the Mill out of plywood and timber over the course of 10 weeks, with the Mill itself (and its walls) standing in for the Houses of Parliament (which were added using CGI).

“We made four houses, but they're designed in such a way that they can play a courtyard,” Montgomery said. “They're painted quite uniformly so that we make them look like they're different locations. So you can shoot down alleyways, sides of buildings, and so on. So you get a sense that it's a much bigger set of London.

“And we've used the idea that the side of the Mill, which we'll CG, is the side of Westminster. Because it was a huge complex. Westminster was a much bigger complex than we imagine.”

The set was then given a distressed, filthy look to match the London of the time, including shipping in specially-mixed mud to cover the ground authentically.

“It's blackened to give it that dirty, lived-in look,” producer Ollie Madden said. “When you really start researching it, it was really bad.”

“I mean, it was squalid to be honest. Because people used to kill their animals outside. It was pretty disgusting. They would throw excrement outside as well, so it was extremely, let's just say it was not a healthy place to be, even though it was THE city of the land.

“[London] was really jerry-built - there were no straight corners, everything sagged and bent. And this was of course before the city was burnt in the great fire [of 1666], it was wiped out. And so very little of it remains physically in London.

“Not much of the country's physical working class or middle class Jacobean or Tudor architecture...you've got all the big stately homes, but you don't have the smaller stuff.”

Lucky there are always production designers on hand to bring it back to life, then.