Harlots uncovered: behind the scenes of ITV's new period drama

Samantha Morton leads the cast of this ITV Encore/Hulu production as Margaret Wells, an ambitious brothel owner who has worked her way up from the streets and now runs a popular establishment – though she wants to move to fancy new premises.

Downton Abbey's Jessica Brown Findlay stars as her daughter Charlotte Wells, who is living under the protection of her "keeper" Sir George Howard (Hugh Skinner), while Lesley Manville is the dastardly Lydia Quigley who owns a rival brothel serving the elite of Georgian society.

"Well, it was quite a challenge, finding somewhere where we could shoot all this stuff, because you are going from St James's type houses to Covent Garden to Nancy's little hovel, and there are limits to where you can find [all that]," Executive producer Alison Carpenter says, taking time out on a winter day to chat in the freezing, high-ceilinged, once-grand room that will soon be transformed into Margaret's back parlour.

Luckily, she and the team – including co-creators and writers Alison Newman and Moira Buffini – struck on Langleybury Mansion, an empty country manor near Watford with the perfect balance of run-down bits and done-up bits to create the whole spectrum of Georgian sex work. It has been used before for TV (you'll have seen parts of it in the BBC's Great Expectations), but this time, the Harlots team took over the whole building.

Downstairs at the mansion we can see the remnants of grandeur, but climbing up the rickety stairs we reach a warren of rooms that are more down-at-heel. "It in the way that you do in these houses, you have ground floor rooms where you have the biggest, stateliest rooms – we're using those for Lydia Quigley's house, and you've got the magistrate's court and the coffee house. And as you go up the house the rooms get smaller," Carpenter explains.

We come to a suite of bedrooms, all ready and waiting for Margaret's girls to use, before exploring the really dingy rooms at the top. A shout comes up the stairs: the cameras are rolling, so we freeze in place on creaking floorboards in a tiny, tatty, run-down chamber like robbers caught in the act (though not the "act" this room will soon be used for by dominatrix Nancy).

Outside, the team has created the streets of Georgian London – Greek Street on one side, Covent Garden on the other – complete with shopfronts and houses and pavements, water-pumps and wagons, and dark passageways perfect for the harlots to vanish down with their clients. A green screen is strung up at one end of the street so the rest of the scene can be painted in.

"We absolutely didn't want it to feel all very interior and dark and fusty, we wanted to feel the scale of London," says Carpenter. Buffini adds: "It was always really important that it wasn't a show that was set in rooms, because London is such a character and it really does come across, this amazing full, vibrant, lively city."

Lunchtime arrives, so we grab some food and head to a trailer to eat with the cast and crew. No matter how many times I hang out on the set of a period drama, I can never quite get over the oddness of seeing actors decked out in petticoats and wigs and sitting down to a nice plate of lasagne and chips as they check their iPhones. They stare at us as if we, not they, are out of place: what is a pack of 21st century hacks doing in Watford's version of Georgian London?

On our way back to the Wells' parlour (press HQ), we walk past a young woman in an elaborate dress, with her hair piled high in a towering wig and a heart-shaped mole painted on her face. It's Jessica Brown Findlay!

Later, Lesley Manville pops into our parlour in the same outfit. "This is a particularly done-up outfit and wig because it's the end of the series and there's a storyline where Jessica Brown Findlay's character and I, I've sort of taken her on board and brought her into my house, and so we've gone a bit twin-like in our looks," she explains, with a laugh. "But this is really, really quite a big look. I'm not going to Tesco, let's put it that way."

Is it comfortable, transforming into an 18th century high-class brothel owner?

"The corsets are quite tight," Manville says. "The wigs are alright, I don't mind the wigs so much, but the make-up's very pale, it's very cakey and it's very ageing because it gets into every little nook and cranny. And so you feel like you're 105 when you're not."

When Samantha Morton pays us a visit, she's looking a lot less fancy. "I play someone who is of a lower class than Lydia. So I don’t really spend my money on myself, I spend it on my girls, making sure they look good," she tells us. "It’s a different world. You’ve got Lydia Quigley’s world and you’ve got Margaret Wells’ world."

So many worlds in just one building – and who would guess? "It looks like it's had twice the budget than it actually has," Buffini confides. "People have just been so imaginative and so clever."

Eight-part period drama Harlots will premiere on Monday 27th March at 10pm on ITV Encore. It will also be available via US on demand service Hulu from 29th March