Sightseers is the story of a thirtysomething Midlands woman, Tina (Alice Lowe), who still lives at home with her anguished mother, Carol (Eileen Davies). Mum is grieving the death of her dog, the mood encapsulated by one wicked retort to Tina: "You're not a friend, you're a relative."

The pained emphasis on the last word of that line really rams home how dysfunctional their relationship has become. However, rather than wanting to repel her daughter, this is a passive-aggressive act aimed at keeping Tina trapped in the family home in perpetuity.

Cue the arrival of the hapless Chris (Steve Oram) and his caravan. Chris's introduction to Tina's life has hastened the reluctant cutting of apron strings - whether Carol approves or not. And so begins a dark descent from sheltered mummy's girl to murderer's muse and active accomplice, via B roads, forgotten tourist destinations and campsites.

Tina and Chris are natural, everyman killers as opposed to born-to-kill nihilists. They're damaged versions of a very British stereotype. Behind closed doors they can be themselves, bringing out the best and worst in each other. However, when they're out in the open they retreat to the habits of repressed, awkward and uptight Brits who shuffle through life hoping not to be noticed. Once on the road, the caravan provides their sanctuary away from prying eyes. The pair are not overtly evil. They just happen to do evil things. Think Fred West carrying out DIY for his neighbours.

Not just the film's stars, Lowe and Oram are also the creators of this twisted couple and authors of the screenplay. They drew on their mutual love of an ordinary Midlands upbringing and tedious family holidays, then juxtaposed those universal traits with the less common one of having a serial killer inside. Originally designed for the stage, the Tina and Chris double act was developed for a TV pilot but ultimately deemed too dark for the nation's broadcasters. Enter executive producer Edgar Wright (Shaun of the Dead, Hot Fuzz, etc) who thankfully saw the cinematic potential.

The final piece of the jigsaw was securing the services of Ben Wheatley to direct it. His previous two outings, Down Terrace (2009) and Kill List (2011), excelled in realism tinged with dark laughs. Qualities that Lowe and Oram's script demands. It's no exaggeration to suggest that within the confines of this green and pleasant land, Wheatley evokes such silver screen treasures as Badlands - just with a little more grey sky and drizzle.

Elsewhere on the Quietus, the director talks about having never seen Mike Leigh's Nuts In May before shooting Sightseers. However, Oram and Lowe acknowledge the influence of the don of homegrown character comedy on their writing.

Tony Way (Down Terrace, Ali G Indahouse) is joyful as the slovenly litter bug at the National Tramway Museum in Crich, while the imposing pomposity of Richard Lumsden at the stone circles will make you think twice about complaining about dog mess again. Richard Glover's graduation to the big screen offers a nice tonal change that makes you forget about the murders for a short while. His character Martin's arrival at the campsite by bike, dragging a carapod (a single sleeper plastic caravan or, as he describes it, "an economic migrant pod"), sparks new life into Chris. However, Tina sees Martin as a wedge between her and Chris - and so begins a rapid descent from creative, self-justified murder to much shakier, irrational killing grounds.

For a film with multiple murders at its core, Sightseers' greatest trick is the warm glow of affection it gives you for Tina and Chris. On the surface they are really friendly and bubbly, but underneath they are yearning to unleash their warped sense of morality on the world. Forget all the hackneyed British holiday references from 1970s cinema, like Carry On Camping or Holiday on the Buses. Imagine instead Bonnie and Clyde, reinvented as a mundane pair of Brummies decked out in Gore-Tex, fleece, walking boots and interesting knitwear. This is a joy of a road movie. Boldly heading out, caravan attached, to celebrate the hinterlands connecting tired towns and cities of the Midlands and South Yorkshire, via a very British killing spree.