In their spare time they started posting musical clips on their Facebook pages simply to show friends and family what they were up to. Then suddenly they went viral. “I remember we had 750 likes and saying wouldn’t it be great if we had 800,” recalls Nicola. The next thing the views were in the millions.

They still don’t know how it happened but they saw the impact of the virtual fanbase when they relocated from Toronto to the UK last summer and appeared on the Edinburgh Fringe. Rosie knows why their month-long run had sold out before they had arrived: “It was all due to The 2016 Song.”

Success has had its downside, though. “I hate the internet. It can really mess with your head,” Rosie continues. “It’s a whole world of despair, it drains you. It’s an ordeal,” adds Nicola. They would read comments, hit “refresh” and immediately see more comments. Not all were favourable, so they stopped reading them. “It’s fair enough when people don’t like something. I don’t like everything but I don’t announce it publicly,” says Rosie.

After Edinburgh they were approached by Nationwide to come up with songs. The idea was that different performers would appear but their work has proved so popular they have fronted all of the ads, with lo-fi tunes ranging from witty ditties about their tiny house — “I can shower in the kitchen sink/have a shave while I have a drink” — to wondering which sibling mum likes best.

Not that working for a building society has helped them get on the London property ladder yet. But it does mean they have been able to focus on comedy full-time. Nicola hopes to launch a podcast: “About unheralded women in history, such as the Countess of Castiglione, who was a pioneer in photography.”

Both are single. They rehearse and write in each other’s houses, preferably at Nicola’s home in Kennington so that she doesn’t have to lug her keyboard over to Rosie in Lewisham. The process varies. “Sometimes it can start with a premise, like people who eat brunch. Sometimes we start with a type of musical genre, such as a Seventies rock ballad. We find a lot of music is inherently funny,” chuckles Nicola.

They chose not to live together to give each other space: “We are very different,” says Nicola. “I’m quiet and like to stay in, while Rosie is sociable and likes to go out. If we’d been in the same year at school we wouldn’t have been friends but our brains work the same way. We are usually thinking the same thing.”

What they also have in common is that they are both stubborn. They resisted pressure to follow The 2016 Song with The 2017 Song. Instead they released Have A Cup of Tea, suggesting everyone forgets about the imminent apocalypse and puts the kettle on (“Have a cup of tea when you’re feeling blue/have a cup of tea when Prince Harry didn’t choose you.”). Rosie: “2016 was just such a weird crazy circus of a year. We couldn’t replicate it so we didn’t want to try.”

They really want to be known as performers, not online sensations. Rosie says: “You get so much more back singing to faces rather than singing to a camera.” 

Their show is called The Kindness of Stranglers. This is not a misprint. There are upbeat numbers asking existential questions such as whether a Big Mac with bacon is still a Big Mac and downbeat numbers about death. “We’ve had emails from parents who want to bring their children because they like our song about bees. We say ‘Well, if you don’t mind them hearing songs about suicide too…” 

Flo & Joan have a monthly residency at Brasserie Zédel, W1 (020 7734 4888, brasseriezedel.com), next show March 30.