And then you decided it would be a musical?

It had something not to do with Jerry Springer the Opera, but the next version of that – I wanted to make something was high brow and low brow. Musicals are good at talking about massive things in quite a jovial way. West Side Story is about racism, Rent is about AIDS. It wasn’t that I was like “I love musicals let’s make a musical”. I actually kind of hate musicals, but it seemed like the right medium.

Did you watch a lot of them to research it? What was the best and what was the worst?

Book of Mormon... I left half way through. Everyone was like "haha racism" and I was like “no". I loved... hmm... London Road, but that’s it really.

How did you research the cancer?

I followed loads of people around. About 15, 20 people. The characters are either based on people I met or an amalgamation of a couple of people. The smoking dad is the only one that's not real. I just really wanted a smoking person in it, as a smoker myself, but we didn’t find someone so I made one up. I don’t usually do that but I thought I was allowed because there’s plenty of people like that around. A lot of the people we got were just friends of friends or people Judith was talking to at her hospital. Everyone knows someone with cancer.

Do you know anyone?

I didn’t before. But still, I thought it would be interesting to make a piece about how I was terrified of it and didn’t know anything about it. And then once I started to learn about it I got less and less terrified of it. It was an experiment to talk to people and see if I could make something out of people’s stories. I got really into the science of it after that.

How did you write the songs?

The songs were written from interviews. With "Kingdom Of The Sick", I knew from Susan Sontag's quote that I really wanted to write a song about that so I asked everyone I met the same question about it. Other songs like "Miracle" came from a woman I met called Lara saying she’d had this fundraiser that was 70s so I knew I wanted to make her a disco banger. And she kept talking about how there must be a miracle cure and she was in denial so I made it about that. She was always terminal when I met her, but only a month before she died did she say, “Right I’ve got terminal cancer”. But that was just her coping mechanism.

The story of Stephen and his mum was really easy – he just told me about how his mum came to hospital and it was really embarrassing and she kept interrupting and saying he should put his sperm in the sperm bank and he didn’t want to do it. And then Shannon was based on a hypothetical situation about having a rare genetic disorder and being pregnant. But her mum had the same cancer so the part about having a predisposed condition was true.

You mention in the musical that, during making it, Frank got sick?

He’s been ill for a while. He has a really severe form of epilepsy so we’ve been in and out of hospital, he was in hospital having 100 fits a day. I spent a lot of time following people around in the kingdom of the sick and then went in myself with him.

Is that how you decided to have the main character as a mother with a baby?

It just sort of evolved into that. There was always going to be a woman who was a guide and she was talking through what it’s like to have cancer but it was just too dry. I was going to be in and then I wasn’t in it.

The casting is really diverse, was that conscious?

We wanted some people in the cast who'd had cancer before. We wanted to have lots of different faces, a cross section of society. Wanted it to look like London. To have people with disabilities. For it to look like ordinary people. And then we had to cast people who could act and sing and dance as well.