'I'm not a saint, just lazy,' explains Alan Bennett (played uncannily by Alex Jennings) when asked about his decision to allow cantankerous drifter Miss Shepherd (Maggie Smith) to make his Camden Town driveway her home from 1974 to 1989. Even as he takes pity on her predicament his thoughts amusingly turn to strangulation, not least as her van's presence acts as a relentless reminder of his neglect of his mother (Gwen Taylor), as Miss Shepherd becomes her 'derelict counterpart'.

The Lady in the Van is an adaptation of Bennett's 1999 play which reunites him with its director, Nicholas Hytner, whom Bennett also collaborated with on the stage and screen versions of The Madness of King George and The History Boys. It's a study of a woman who's fled her past and is hiding in plain sight, camouflaged only by her ignominy and stay-away smell (described brilliantly by Bennett as an 'odoriferous concerto'). Smith cultivates great sympathy for the irascible Miss Shepherd, who's plagued by painful memories and whose eyes are often nervously downcast, allowing us to clock the vulnerability behind her self-preserving hostility.

Bennett's screenplay is a delight, routinely enriched by his masterful and richly comical turn of phrase. Commenting on Miss Shepherd's move into his drive Bennett says, 'She applied the handbrake with such determination that, like Excalibur, it could never be released.' By offering us not one but two (bickering) Bennetts – the writer permanently in situ, pen scratching away, and the man out there doing the living – the film is able to explore the creative process, and directly address Bennett's personal anxieties. The device also flags him as an unreliable narrator, as we're shown how he fills in the blanks of Miss Shepherd's life. This colourful conceit adds energy but it's also distractingly contrived. Cameos from what feels like the entire cast of The History Boys are a touch off-putting too, and the flight-of-fancy / pulling-the-curtain-back ending is overindulgent.

Whether or not it needs the bells and whistles, the pair's curious story is undoubtedly worthy of cinematic elevation. Shot in the very house on the very street where all this took place, the film shows how the rich and poor of London rub together in a way that's not always harmonious but that has the potential to be heartwarming – a community's capacity for picking up the pieces when societal safety nets fail, even when such kindness is hilariously begrudging. In short, it has an admirable handle on what it is to be British.

Screening on Tue 13 and Wed 14 Oct as part of the London Film Festival 2015. General release from Fri 13 Nov.