Barts Speakeasy Bar in Chelsea: Password Entry, Open Wed–Sun

Barts is a 1929-themed speakeasy hidden inside Chelsea Cloisters, 87 Sloane Avenue, London SW3 3DW, and it is open Wednesday to Sunday from 5pm. Cocktails sit between £15 and £17, a Moët and Chandon glass is £17, and the password arrives with your booking, so you cannot simply stroll up and ask. Barts calls the room the apartment of a Chicago gangster named Uncle Barts, and that conceit runs from the unmarked black door to the cocktails served inside antique books.

Behind the door is a flat dressed like a bootlegger's hideout, with dim lamps, dark wood and a back garden the bar calls the Agave Garden. Everything about the evening is built around not being found, which makes it one of the more theatrical speakeasy bars in London and a sensible first stop if you want the idea done properly rather than as a gimmick bolted onto a hotel bar.

Entry is by booking. The bar's own FAQ answers the password question with a wink: make a booking and the team will send everything needed to gain entry to the apartment. Table bookings, late-night guestlist and private parties are the three options on the booking panel, so pick the one that matches your night.

The story on the bar's website places the entrance at the back of the building, an unmarked black door on the ground floor of Chelsea Cloisters, a mansion block that sits on Sloane Avenue and that the story describes as South Kensington's Chelsea Cloisters. Allow a few minutes to find it, because nothing outside announces the bar. That is the point, and the staff expect first-timers to be a little lost.

Barts is for guests aged 21 and over. Dogs are not allowed unless they are assistance dogs, the bar says it cannot take hen or stag parties of any kind, and because of the tucked-away entrance it is not wheelchair accessible. If any of that rules out your group, better to know before you head to Sloane Avenue than at the black door.

The official site lists the week like this: Monday closed, Tuesday closed, Wednesday 5pm to 1am, Thursday 5pm to 2am, Friday 5pm to 2.30am, Saturday 5pm to 2.30am and Sunday 5pm to 12am. That makes Wednesday and Sunday the calmest evenings and Friday and Saturday the loudest, with the room filling steadily after 9pm.

On Thursdays, Fridays and Saturdays the bar runs Speakeasy Sundowners, its late-night 1920s parties from 11pm until close, and its website adds the line that walk-ins are always welcome at those parties. If you want a seat and a conversation, arrive between 5pm and 8pm. If you want the party, plan for 11pm and expect to dress the part.

Special dates for this season are already on the calendar. Halloween Speakeasy Sundowners run on Friday 30 and Saturday 31 October from 11pm until close, the Espresso Martini Summit runs until 15 November, and the Gatsby-style New Year's Eve party on Thursday 31 December runs from 9pm to 3am.

Inception Group, which owns Barts, records that the bar opened in February 2009 as the company's first venue, and describes it as London's first speakeasy hidden within a Chelsea apartment block. The group says it opened in the middle of the recession with no street presence at all, and that a buzz on Twitter, which had just launched, helped the room find a loyal following. Seventeen years on, the black door is still the only advertising.

The fiction on the website is more fun. Uncle Barts is a boxer turned right-hand man to Al Capone, a smooth talker with a Cheshire-cat grin who fled Chicago on a freighter and bought a quiet ground-floor flat in London. The staff are billed as the Shady Associates, and the website's FAQ signs off with an invitation to get in touch with Big Frank if you are still bamboozled.

The cocktail menu is sorted by mood rather than spirit: sweet and fruity, sour and refreshing, rich and woody, smoky and spicy. Prices on the current menu run from £15 to £17 for single serves. Sticky Fingers is £15.50 and mixes Bombay Sapphire gin with lemon verbena liqueur, strawberry shrub and lychee juice. Smash and Grab at £15 pairs Bacardi Caribbean Spiced rum with Aperol and orgeat.

For a splurge, Put On The Posh is £17 and tops Belvedere vodka, hibiscus and green tea liqueur and passion fruit purée with Moët and Chandon Impérial. For two people, Mary Polly Carr is £32 and is built on Ketel One vodka, Earl Grey tea syrup and sparkling wine. Many drinks arrive in oddities such as antique books or cigar boxes, with smoke, fire and the occasional bubbling flourish.

On the savoury side of the sour section, Tea Time with The Brummies is £15.50 and uses The Botanist gin, Italicus bergamot liqueur and Earl Grey tea syrup. El Patrón at £15.50 uses Patrón Silver tequila with prickly pear liqueur and grapefruit sherbet. In the rich section, Take a Leaf from Uncle Barts is £16 and leans on Monkey 47 gin and Bénédictine, while King Of The Races is £15 and finishes with a tiramisu foam.

A skip, honestly, is the beer list. Camden Hells is £7.30 and a 33cl Madri is £7, perfectly fine but you did not hunt for a hidden door for lager. The non-alcoholic versions of several cocktails are £12, so a driver or a friend avoiding alcohol still gets something built rather than a soda. Small plates come from Gaucho, which helps if you arrive hungry.

Barts asks guests to dress smartly, with smart-casual the ideal. Full 1920s fancy dress is not required, but a touch of vintage flair is welcome. Sportswear, tracksuits and overly casual clothing are discouraged, and the bar says entry may be refused if attire is judged too casual. Trainers that look like gym shoes are the likeliest problem, so wear leather or a clean pair of plain sneakers.

Treat the room as a small flat rather than a bar. People talk quietly early in the evening, the staff will talk you through a menu you will not instantly understand, and the more theatrical drinks are meant to be photographed once and then drunk. Staying kind to the Shady Associates earns the best recommendations.

Sloane Avenue runs between the Brompton Cross end of the Fulham Road and the King's Road, so the neighbourhood suits an early walk. Michelin House at the Fulham Road end is one of the area's most photographed Art Nouveau buildings, and Sloane Square and South Kensington stations are the two nearest Tube options, each a pleasant stroll away through quiet Chelsea streets.

If you arrive early and the black door is not yet open for you, wander the mansion blocks around Chelsea Green and the shops of Brompton Cross, then come back at your booking time. If Barts is full on a Friday or Saturday, a Wednesday or Thursday early evening is the best fallback, because tables are easier and the bar still feels like a secret.

Barts is at Chelsea Cloisters, 87 Sloane Avenue, London SW3 3DW. Open Wednesday 5pm to 1am, Thursday 5pm to 2am, Friday and Saturday 5pm to 2.30am, Sunday 5pm to 12am, closed Monday and Tuesday. Age 21 and over, smart-casual dress, no wheelchair access, no hen or stag parties. Book ahead so you receive the password and the entry details.

If you plan a speakeasy crawl across London, ask Karpo in the Karpo APP which hidden bars are open on the night you are free and which ones pair well with dinner nearby. It is the quickest way to build an evening around Barts without spending the first hour looking for a black door.