Here’s a film about a guy who likes six sugars in his tea – and the viewer might also need a bit of a sweet tooth. It is an amiably daft and sentimental Britfilm, a comedy of the underdog starring Mark Rylance, based on the strange true story of Maurice Flitcroft, a Barrow-in-Furness shipyard worker and amateur golfer who took up the sport in middle-age. Flitcroft practised on the beach, and became known for cheekily entering the British Open golf championship in 1976 as a self-declared professional, thus circumventing the handicap requirement for amateurs. He found himself competing with the likes of Seve Ballesteros, but chaotically chalked up the worst score in the tournament’s history, to the spluttering rage of the puce-faced, blazer-wearing gentlemen in charge. They tried to ban him but he kept on gatecrashing competitions with wacky disguises and fake names.

This movie about him has been written for the screen by Simon Farnaby – based on the 2010 book of the same name that he co-wrote with Guardian writer Scott Murray – and directed by Craig Roberts. Flitcroft is played by Rylance as an eccentric holy innocent; his long-suffering and affectionate wife is played by Sally Hawkins; and Rhys Ifans is the pop-eyed, pompous club secretary.

Now, maybe this casting feels a bit inevitable, and maybe the Britfilm template itself feels a bit inevitable. We’ve already had a movie inspired by Eddie the Eagle, whose entry into the Winter Olympics as a ski-jumper in 1988 depended, like Flitcroft’s escapade, on exploiting a loophole in the rules. Dream Horse was a more heartfelt true story about a community that bought a Welsh Grand National winner. It could be that this film downplays Maurice’s life as a prankster and hoaxer and skates over the stranger and more anarchic side of his personality, playing up instead the underlying sincerity of his dream of becoming a golfer, and his hurt feelings when the press and media mocked him. Or this may be nothing more than the truth. It could well be that Flitcroft brazened it all out and made a show of his outrageous exploits to mask how upset he was with not really being good at golf.

The script takes up an interesting point from Maurice: why shouldn’t the British Open be like the FA Cup, where little clubs are allowed to take on the big names? Could it be that British golf is a festival of class entitlement to which Maurice was not admitted, and that maybe his interest in the sport needn’t have been a big joke? Maybe he had a real talent, which could have been nurtured if things had been different?

Maybe. Rylance is good casting as Maurice: his delicate sing-song voice and sometimes faintly unfocused gaze fit nicely with our hero’s lovably awkward determination, as well as Flitcroft’s sense as a natural comedian that there is something more than a little absurd in the game of golf.

• The Phantom of the Open screened at the London film festival and is released on 18 March in the UK.