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You're too young to have watched the original series but do you remember anything about it from your childhood?

I was quite young when the series first began, but my parents went out a lot at weekends and we had a lenient babysitter! From the third series onward, I didn't miss a single episode. What struck me most was the masterly depiction of the First World War and its aftermath, all seen through the lens of life at 165 Eaton Place. It was actually hugely educational, and sparked my passion for social history. However, when I rewatched the entire saga as an adult, I was surprised at the risqué nature of many of the storylines. It was all going on in Eaton Place, but I was so innocent that adultery, illegitimacy and homosexuality had all gone completely unnoticed!

Did you have fun creating such rich and lavish characters?

Creating and introducing the new inhabitants of 165 Eaton Place proved an engrossing and rather emotional experience. Like every other fan of the show, I have vivid memories of the Bellamys and their staff, and was aware that there were ghosts to cherish, and dispel. Downstairs, there were obvious roles that needed to be filled – a house like 165 needs a butler, a cook, a maid or two, a footman and perhaps a chauffeur. And that, in turn, demands a family upstairs with high standards and plenty of money.

But I never wanted just to clone the characters that had gone before. From the start, it felt right to have a younger couple at the helm of the house, and Sir Hallam's job at the Foreign Office was an obvious way to bring the tumultuous politics of the 30s centre stage. But the most exciting thing about writing drama is that once the broadest facts are in place, little unforeseen details about the characters start to spring up in a very organic way – for example the fact that Sir Hallam and Lady Agnes have been unable to have children, and so treat her younger sister Lady Persephone as a sort of surrogate child – a plan which hits the rocks at once!

Downstairs, putting the staff together proved rather challenging and piquant, as by 1936 servants were much harder to get hold of, and employers had to take what they could get! So I felt at liberty to put together a team of rather wonderful juveniles and eccentrics, some of whom would rather be anywhere else!

And, of course, the great thrill was to bring back the iconic character of Rose Buck. None of this would have been possible without Jean Marsh. Jean IS Rose, and Rose IS Upstairs Downstairs. Rose is now in her 60s, and not as robust as she was – but work is bred in her bones and she will keep on till she drops. I found this profoundly affecting, and sort of had my arm around Rose throughout the writing process. She is at once a unique and universal character – a principled, proud, sometimes vinegary spinster, who feels things deeply and never gives up. I loved Rose; I love her now. I hope it shows.

The series is set in a very tumultuous period. Your scripts very much reflect the external events of 1936 and how these events affect the residents of 165 Eaton Pl. You must have done some extensive research before and during your writing of the episodes?

I am passionate about history, especially when it is about the world 'writ small' – hingeing upon all the details of ordinary people's lives and experiences. I have a large collection of magazines, ephemera and housekeeping manuals, dating from 1803 to 1939, and these are always my first port of call when starting work on a period piece. Of course, the bigger points of history require heavyweight research, and for Upstairs Downstairs I spent some time in serious reading, again using many books from my own library. I do have my own researcher, Joanna, who can ferret out the most extraordinarily obscure pieces of information at short notice eg 'Help! I need a 1936 brand name for a really posh gas cooker!'

Once the writing of the scripts is underway, script editor Liz Kilgarrif double-checks every detail, and can source things like the precise wording of a radio broadcast on a specific date in the story. We also engaged a historical consultant, Juliet Butler, to make sure we had an academic overview.

What do you think it is about Upstairs Downstairs that people love so much? Is it the relationships, the characters, the storylines?

I think the lasting power of Upstairs Downstairs is rooted in its intimacy – it is about a very close-knit group of people who depend on one another for all things, but could not be more different. There are clashes of personality upstairs as well as down, but none of the individuals involved could last a day alone. To my mind, that is the very stuff of life, and we can all relate to it – but the world these characters inhabit is very different to ours, and the drama is therefore seasoned with escapism.

There is also a very strong message of equality in Upstairs Downstairs, which is often overlooked. Because the drama treats both rich and poor with an even hand, it says to the viewer 'Whether you drip with jewels and are swathed in furs, or are wrapped in a pinny and scrub until you hands bleed, your story really, really matters'. It respects and celebrates every aspect of human experience, and that is a powerful thing.

Did you have to change anything about the format of Upstairs Downstairs to tailor it for today's TV watching audience?

We were extremely keen to reference the tone and style of the original, and conferred closely with the series co-creators Jean Marsh and Eileen Atkins, and original producer John Witney, in order to make sure we got this right. This is a sequel, not a remake, or a rehash, and we wanted to preserve as much of the magic as we could. A great deal of the power of the original stemmed from the fact that so much of the drama was sparked by, and played out within, the walls of 165. We set strict limits on the amount of time we spent outside the house, and made sure that any exterior scenes – such as the spectacular recreation of the Cable Street riots – were justified.

It is all about finding a balance, however. In the original series, there was a tendency towards very long scenes that might feel slightly theatrical and slow to a modern audience, so we recalibrated this slightly, whilst striving to keep the drama detailed and satisfying. We also make use of incidental music, which wasn't a feature in the original series, and the sumptuous attention to visual detail is also new.

Do you think this new series will attract a new generation of Upstairs Downstairs fans?

Fans of the original are guaranteed a warm welcome, but there is a huge amount in it for newcomers too. We start with the arrival of brand new owners, and everything feels very fresh and vibrant. Eaton Place in 1936 has a real sweep and glamour – there are fabulous frocks, intense friendships, doomed love and, underneath it all, the cut and thrust of a very dangerous time in politics. Our characters range from Ivy the parlourmaid, aged 15 and reared in Barnardo's to Maud, Lady Holland – doyenne of the Raj and well into her 70s. The old series had fans of all ages and from all walks of life, and the sequel looks set to attract the same.

As an ensemble, do the cast work well?

Upstairs Downstairs is only partly about class; it is very much about family, and the way in which we relate to the people around us. So from the start, it was not just the individuals that were important, but the way they would connect with one another. And the actors really did gel, from the moment they first sat down and read the scripts together. Although I found it very funny, when – halfway through filming – I discovered that all the 'Upstairs' characters were in one hotel and all the 'Downstairs' characters were in another.

The sets are exquisite. What emotions did you experience when you first stepped on to them?

Never mind the sets – I cried when I saw the model! Set designer Eve Stewart likes to start with a doll's house style mock-up of the eventual design, and when I saw it I suddenly realised that yes – this was really going to happen. Upstairs Downstairs was really coming back. When finished, the sets literally took my breath away – everything was built on such a huge scale, and yet there was so much detail. They are also incredibly solid. I must confess that once or twice I sneaked into the entrance hall when nobody was looking and ran up and down the stairs pretending it was my house. I'd quite like to live there actually. We should rent it out for weekend parties!