I am in Mark Hampshire and Keith Stephenson’s bright, plant-filled sitting room, enjoying a crash course in midcentury design. Exhibit A: Robin Day’s 1964 Forum sofa, pioneering in its mix of oak and steel. There are folkish wooden dolls by the architect and designer Alexander Girard – “another hero of ours”. Next, a prized set of tumblers made as commemorative souvenirs for the New York World’s Fair of 1964. “It was a showcase for midcentury architecture and design: the last hurrah for the American dream before the realities of the Civil Rights Movement and the Vietnam War set in,” says Mark, warming to this theme. “Each one is a piece of social history.”

We’d always imagined ourselves living on a 1960’s estate with a shared garden and garage for the Mini

They wish their south London home was an example of enlightened, postwar architecture, too. “We’d always imagined ourselves living on a 1960s estate with a shared garden and garage for the Mini, that’s how geeky we are,” says Keith. But modernist houses are rare so the couple, co-founders of interiors business Mini Moderns, settled for this contemporary townhouse instead. Tucked behind a Victorian terrace, on the site of a former print factory, the three-storey property is part of a development of 15 live-work houses designed for the creative businesses – photographers, artists, designers – who give the mews its communal feel. Flowers and sculptures sit outside front doors; in summer the ground-floor double-height studios turn into party venues. “We like the friendly atmosphere. We’ll water our neighbour’s lavender when she’s away and friends take in deliveries if we’re out,” says Keith.

‘Still in production’: ‘String’ shelves in the living area. Photograph: Rachael Smith/The Observer

From the studio, we head upstairs to the three-bedroom duplex where Burt Bacharach drifts from the speakers and light streams in through the wide windows. “When we moved in this was a featureless, magnolia box with horrible surfaces. So we said right, let’s eradicate the worst bits and give it a 1960s townhouse vibe,” says Mark. Out went the carpet (“a cat magnet”), replaced by wooden floors painted grey for a vintage Linoleum effect. New sliding doors lead to the compact, monochrome bathrooms.

The couple insist they are not purists (a bank of sleek fitted wardrobes is from Ikea), but almost all the furnishings here have provenance. In the bedroom, George Nelson’s 1948 Bubble pendant glows next to a chair designed by Ernest Race for the Festival of Britain in 1951. Inspired by another design idol, Gio Ponti, they also designed the floating ply bedside tables and strikingly tall headboard that adds to the streamlined feel.

All mod cons: 1950s fabric in the bedroom. Photograph: Rachael Smith/The Observer

Initially, they painted everything white, but gradually the house has become a testing ground for their midcentury–feel designs. “If we can live with them, then at least we know other people can, too,” says Mark. The pattern-loving couple grew up in Yorkshire and all their designs spring from the “warmth” of shared memories. “We never met as teenagers, but we did all the same things. We watched 1960s films on Channel 4. At weekends we went to junk shops and spent our pocket money on 1950s wire fruit bowls or painterly curtains which we’d turn into shirts for Saturday nights out,” reminisces Keith.

The blue and white waves of their “Whitby” wallpaper evokes summer holidays on the Yorkshire coast (“Spain hadn’t been discovered then”), while “Art Room”, adorned with oil lamps and African masks, pays homage to sixth-form still-life classes. In the hallway, graphic “Pluto” (inspired by dazzle camouflage) clambers over the walls and ceiling, adding to the feel that you have tumbled into an Op Art painting.

Shelf life: jumble-sale finds on display. Photograph: Rachael Smith/The Observer

When they launched in the mid-1990s, some of the duo’s earliest clients included the Southbank Centre, Radio 6 and John Lewis. Now their vibrant Paisley design, inspired by Hanif Kureishi’s Buddha of Suburbia, hangs in the permanent collection at the Whitworth Art Gallery in Manchester.

Wallpaper also brings colour to the staircase, which rises to the top floor. Like that yearned-for 1960s house, this is the main living area: kitchen, dining area and sitting room flow towards the long terrace. Here, walls were removed to open up the boxy landing. “It didn’t take much to make the place feel much sleeker,” says Keith. New doors, in a mix of plywood and paint, transformed the developer kitchen into their design ideal: “Frank Guille’s 1958 kitchen for the British manufacturer Kandya to be precise.” The G-Plan sideboard belonged to Keith’s mum (“I was too late to save the rest, which went on the bonfire,” he laments), the clunky 1970s Trimphone still rings “very loudly” and the compact Danish dining table, bought at the annual Midcentury Modern Show in Dulwich, southeast London, flips open to seat eight: “Another example of good design.”

‘Another example of good design’: Danish dining table from the Midcentury Modern Show. Photograph: Rachael Smith/The Observer

That teenage compulsion to rummage through market stalls for overlooked gems has never left them. Pocket-money finds – Susie Cooper cups, orange-spined Penguin books – have been joined by more sophisticated purchases, such as 1970s West German studio and sculptural crockery, designed by Susan Williams-Ellis for Portmeirion in the 1960s. “We only collect olive green.” Their favourite pieces are displayed on the flat-pack “String” shelving which has travelled with them from previous, rented homes. “The system was designed in 1949 by husband-and-wife team Nils and Kajsa Strinning for a publisher who was looking for an affordable, easy-to-install way to display books,” says Keith. “It’s never been out of production since – which is just the kind of anecdote we like.”

Mid-Century Modern Living, by Keith Stephenson and Mark Hampshire, is published by Kyle Books (£20). Buy a copy for £17.60 at guardianbookshop.com