Almost eight years after his critically lauded debut feature, Attack the Block, writer/director Joe Cornish is finally back with Arthurian family adventure The Kid Who Would Be King. Starring Louis Ashbourne Serkis (son of Lorraine Ashbourne and Andy Serkis) as Alex, an unlikely 12-year-old heir to the fantastic dynasty, it's a project that Cornish has been working on since 2012, although he claims to be no expert on the mythology of King Arthur.

'The truth is I'm not obsessively knowledgable about Arthurian legend,' he admits. 'I like the movies. I liked Disney's Sword in the Stone, I saw John Boorman's Excalibur when I was much too young for it so it had a big impression on me. I like Monty Python and the Holy Grail but that's it. I never read Le Morte d'Arthur, I don't think I even read The Once and Future King. I just knew the things that everybody knows about Arthur which is the Sword in the Stone, the Round Table, the Lady of the Lake, Merlin and Camelot. That's sort of all you need to know, and they're incredibly indelible, dramatic devices that have lasted for hundreds and hundreds of years.'

In The Kid Who Would Be King, hero Alex is accompanied on his grand adventure by best friend, Bedders (Dean Chaumoo), and the pair form an uneasy alliance with school bullies Lance (Tom Taylor) and Kaye (Rhianna Dorris). Theirs is an interesting arc, not entirely dissimilar to the way Cornish treated his flawed human heroes in Attack the Block. 'I suppose I'm an optimistic person,' he explains. 'I also don't really like violence between human beings in movies. I quite like watching it but I find it weird to write or direct. So, consciously in my movies all the action and conflict takes place between human beings and fantasy creatures. I was bullied a bit at school and, actually, the people that bullied me are now very nice people. I don't think they were showing some sort of evil in themselves. And I've bullied a bit, as well, because what goes around comes around. And it's not good to bully but often you're being buffeted by psychological winds that you can't control at that age.'

For a family film, it features some pretty hair-raising moments, each of which Cornish considered very carefully. 'I think it's quite important for kids movies to be a little bit scary,' he reasons. 'Darth Vader scared me, some of Ghostbusters scared me when I was 15. One of the things that all of those 80s kids movies have in common is that the stakes are real: the fantasy happens in the real world and there's genuine threat which means that, when the heroes overcome it, it feels more satisfying than if it's animation or something too fantastical.'

While much of his best-loved work has involved performing (most notably with childhood friend Adam Buxton on Takeover TV, The Adam and Joe Show and their much-lamented 6 Music radio show), Cornish has always been more comfortable on the other side of the camera. 'With the Adam and Joe Show I was more the director. I was just jealous of Adam being on camera so I sort of forced myself to sit next to him,' he laughs. 'I'm too sensitive, I think. I giggle a lot, I cannot hold a straight face. Adam's such an amazingly good comic performer: his impressions are amazing, his voices are amazing. He's the funniest person I've ever met in my life, and he can make me laugh harder than anybody else I've ever met. So I don't want to be in competition with him.'

Although it's taken Cornish the best part of a decade to ease back into the director's chair, he's been busy behind the scenes all this while. He spent some time on aborted spy thriller Section 6, co-wrote Steven Spielberg's The Adventures of Tintin: The Secret of the Unicorn ('the experience of a lifetime') and was in speculative talks over Star Trek 3 and Die Hard 5. 'I was approached about them and then I took a meeting or two but what happens is that as soon as you take the meeting a story appears in the movie industry press saying, "Joe Cornish attached to this or that!" but you're not really attached, you're just taking the meeting. And if you think that you might be getting out of your depth and step away then the story becomes, "Joe Cornish fails to land Star Trek!" but what's actually happened is you've taken an investigatory meeting and sometimes you've decided yourself that you're not right for it. So that can be frustrating: the way it's reported. But what are you gonna do? Call up Variety and say, "Withdraw that story!" Nah, you just let it happen really.'

But one instance of creative differences really did cause a stir. In 2014, fellow writer/director Edgar Wright was all set to direct Ant-Man, a project he'd laboured over for many years, using a script he'd co-written with Cornish. Just as the cameras were about to roll, the pair quit the project under a dark cloud. Cornish is now sanguine about the experience. 'At the beginning, people didn't really respect superhero movies and they hadn't really succeeded at the box office. Marvel's agenda was to bring auteur directors in to try to elevate the material, hence Ang Lee's Hulk, and attaching someone like Edgar who's a very idiosyncratic director,' he explains. 'By the time we actually came to start pre-production on the movie, the company's approach to its material had changed a lot. They were extremely successful, they had a formula and they were very interested in cross-pollinating their different characters, starting to build up to what we now know as the Marvel Cinematic Universe, and it's been incredibly successful for them. The script that Edgar and I had written was not like that. It was a stand-alone, very carefully thought through action heist thriller. There's a bunch of stuff in the finished movie that is ours, a bunch of stuff that isn't.'

If only Wright and Cornish had entered the fray a few years later, it might have been a different story altogether. 'There was a tug of war between (president of Marvel Studios) Kevin Fiege and what was called the Marvel Creative Committee, so Kevin didn't have autonomy; he had to answer to this group of brand managers above him.' Ultimately Fiege won his battles and took control of the MCU, empowering him in recent years to hire marvellously idiosyncratic directors such as Taika Waititi for Thor: Ragnarok, a decision which resulted in one of the MCU's greatest critical and commercial success stories. Cornish has since seen the Peyton Reed-directed Ant-Man. 'Oh yeah, many times. I like it. It's a fun, enjoyable film. But it's not Edgar's version. For me it was a privilege to be involved in it, and an amazing thing to be a part of.'

As for his next move, Cornish isn't quite sure. His unfilmed feature adaptation of Neal Stephenson's sci-fi novel Snow Crash has turned into a potential TV series ('It's a big project, it needs to be done right') and he has another script on the go. 'I'd love to make another film. I'd love it to be some sort of fantasy action adventure,' he laughs. 'But we'll see. I feel very lucky – it's sort of a miracle this movie got made because it's not the sort of film they make any more.'

The Kid Who Would Be King, general release from Fri 15 Feb.