There is a very good concept at the core of Downsizing, a kooky alternative to the Black Mirror gloom that is so prevalent in modern sci-fi. Scientists have found a solution to the problems of overpopulation, and that is to shrink people down, so that they are a mere five inches tall. The film runs with this for a fitfully fun, though in the end, frustrating sci-fi epic.

Our entry into the world of Downsizing is Matt Damon's Paul Safranek, an occupational therapist whose life seems to have come unstuck. When he meets a former High School friend who has gone through 'cellular miniaturisation' it opens up a world of possibilities for him and his wife. Soon they find themselves with shaved heads and ready to journey to a smaller, seemingly better world.

This would seem strange material for director Alexander Payne. His work has oscillated from savage satire (Election is a must see) through to acidly-funny, sharply observed character dramas (Sideways & The Descendants). Certainly this is his highest concept film to date (although he, bizarrely, co-wrote Jurassic Park III) and may explain why it feels like the film isn't quite sure where to go with its own concept. Early scenes of a bald Matt Damon going through the process of downsizing are genuinely funny, and it has to be said the CGI is remarkably good here. But a pair of rather sudden plot developments in the second half mark a move away from the central idea and make the film a stranger proposition, almost uninterested in its original storyline. It feels like it's missing opportunities when it comes to exploring the potential of the world it has created, and almost feels made for TV, where it would have room to breathe.

Part of the problem of Downsizing is just how fiercely dull our lead character is. Matt Damon is fine but lacks the charisma that made him such an effortless delight in the likes of The Martian, the Ocean's movies and True Grit. His character is closer to a cipher for audiences, and thus leaves the viewer underwhelmed. Damon is ably supported by the likes of Christoph Waltz and Kristen Wiig, but it is Hong Chau who upstages everyone. Her character is much more intriguing and Chau magnificently handles the role. She really should be the focus.

If I sound disappointed with Downsizing it is because I am. Alexander Payne is one of the finest directors currently producing films in American cinema, and the concept here is a great one. Yet the film can't seem to work out how to use it. There is a decent heart here, emboldened by a stellar turn from Chau, who is a lot of what makes Downsizing continually engaging, even when it frustrates.

This is a London Film Festival preview and Downsizing will be released on Friday 19th January 2018.