As the son of Liverpool legend Cilla Black, I expected Robert Willis to have a trace of his mum’s famous scouse accent. This was not the case. Instead, Robert sounds very BBC British as he tells me proudly about his mum, and his stage show Cilla: The Musical.

Cilla Black is perhaps best known to us millennials as the charming and charismatic host of Blind Date, the original Saturday night dating show. What is lesser known is that she enjoyed an exciting and incredibly successful singing career, achieving two number one singles in 1964 and eleven subsequent number ones between then and 1971. Perhaps her most famous single, Anyone Who Had A Heart, was the UK’s biggest selling single by a female artist in the 1960’s. Oh, and she was childhood friends with John Lennon.

Despite growing up with a British icon for a mum, Robert is quick to tell me that he and his brothers enjoyed a normal childhood. “Her fame was part and parcel of our upbringing,” he reveals. “I was born in 1970 when she already had a successful pop career, so it wasn’t like she became famous. Our parents wanted to make sure we had as normal an upbringing as possible, so we weren’t spoiled. They made it obvious to us at a very early age that what they did was their work, and that they were parents when they were not working. They wanted to spend as much time as possible with their family, which was what was most important to them.”

Robert’s father – also called Robert, though better known as Bobby – was a songwriter himself, before becoming Cilla’s manager. When Bobby died in 1999, Robert took over the role.

After enjoying a television career spanning over a remarkable forty years, Cilla and Robert were approached by screenwriter and television producer Jeff Pope, who was looking to create a television series that told the story of Cilla’s career. Ultimately, this is not what the ITV mini-series, Cilla, which aired in 2014 turned out to be. “It ended up being about her pop career, and finished at the start of her TV career, which is the part that everyone knows about. But no one knew the actual details of her music career, or the details of my parent’s relationship.”

Initially, Robert tells me, Cilla had some qualms about seeing her early life dramatized for a mini-series. “At that point, the model for TV dramas was revealing salacious or unsavoury details, whereas this wasn’t going to be like that. What we were doing was telling the love story between my mum and my dad, and the story of her ambition. We weren’t looking to deconstruct her character. And when she understood that, she let us get on with it. She trusted me, and she trusted Jeff, and she knew we would do the upmost to do the right thing: tell her story.”

And Cilla was right to trust Robert and Jeff; the series enjoyed a great reception from the British public, and indeed, from Cilla herself. “She didn’t want to watch it initially. She was worried she wouldn’t like it. But we sat down together and watched the whole thing. To start with she was like ‘hang on a minute, our front door wasn’t that colour!’” Robert laughs. “She was noticing those initial things – it was her life after all. But she got over that after about ten minutes and just loved it. She was just enjoying the drama.”

After the success of the series, Robert and Jeff began to wonder if the story and the music would work for a stage show and began working on a first draft. “Mum was very much in favour of that!” Robert tells me. Sadly, Cilla passed away at her holiday home in Spain at the age of 72, in 2015, before the musical premiered.

“Initially after her death, it would have been easier not to do [the musical].” Robert reflects. “If the TV drama hadn’t worked, it wouldn’t have been the end of the world… But if we’d done this show and it hadn’t worked, it would have been heart-breaking. But the risk was worth taking. Partly because I’d lost both parents; the show isn’t just about my mum, it’s about my dad as well, and doing right by their memory was a big pressure. It was something I promised her I’d finish; it was a labour of love. And the payoff has been a fantastic show that entertains and moves people. That was what she was about.”

Robert is extremely proud of the musical and the TV series, but he is also clearly relieved that they have been received as well as they have. “A lot of these shows, the majority of them actually, don’t work out. But thank God it did for us. I was a bit reticent before it opened; I knew we had everything ready, that Kara [Lily Hayworth, who plays Cilla] and the rest of the cast were excellent. But as much as you think you have all the ingredients until you see it you just don’t know. It was the same when the TV drama came out on DVD. I said, you know what, if people don’t get it, that’s ok, I can live with it. I’m proud of it. Thankfully, everyone got it. But you never want to be that presumptuous.”

It’s not a jukebox musical. The drama is there to go with the music. The music empowers the drama, and the drama empowers the music.”

And Robert’s favourite music from the show? This is the question that stumps him. “I’ve got lots of favourites, but no real one. Anyone Who Had A Heart is a lovely dramatic moment. The first time I saw it, it was outstanding. And Liverpool Loverboy, as she used to sing that to us as kids. But I guess it would have to be Anyone Who Had a Heart.”

Robert leaves me with something that hadn’t yet occurred to me, but helps me understand better exactly why he is so proud of his mum and her achievements. “Women [in the 1960s] – and today – tend to be judged on how they look, and not so much on what they have to say, or what their talents are,” Robert says thoughtfully. “Whereas Cilla’s force of personality and character were what came through. And I think, in that respect, she was certainly ahead of her time. Although things are much better now, we still have a long way to go. But she certainly did her bit.” Cilla: The Musical will be at Norwich Theatre Royal 17-21 April