Germany
Slapp Happy was formed in 1972 in Hamburg, Germany by British experimental composer Anthony Moore. Moore had recorded two avant-garde/experimental solo LPs for Polydor Germany, but they rejected his third because it was not commercial enough. As a result of the rejection, he proposed a pop project with his girlfriend (and soon to be wife), Dagmar Krause from Hamburg, and a visiting American friend, Peter Blegvad. At the time, Krause couldn't sing because of problems with her voice, but when she heard Blegvad's singing she agreed to sing for the group.
With krautrock group Faust as a backing band, Slapp Happy recorded a debut album, Sort Of, for Polydor Germany in 1972. The songs were simple, primitive pop, a "naive rock" as Peter Blegvad put it. Commercially, the LP did not go very far, primarily because Slapp Happy refused to perform live.
In 1973 they returned to the studio (again with Faust as their backing band) to record their second album, Casablanca Moon. After the commercial failure of Sort Of, Polydor had demanded more pop-sounding material, and so Moore and Blegvad wrote "straight" pop songs, but Polydor was still not happy and refused to release it.

England
Slapp Happy then left Polydor Germany and moved to London where a record deal was signed a deal with the then emerging Virgin Records label, which was looking for experimental groups. Faust and Henry Cow had already signed up. At Virgin's Manor Studios in Oxfordshire, Slapp Happy re-recorded Casablanca Moon with the help of session musicians (under the direction of violinist Graham Preskett) and Virgin released it as Slapp Happy in 1974. It was not until 1980 that Recommended Records released the original Casablanca Moon (with Faust) as Acnalbasac Noom (the words of the original title written backwards). Comparison of the two releases revealed two very different musical arrangements. Acnalbasac Noom had a raw and unsophisticated feel about it, whereas Casablanca Moon tended to be more sentimental with more complex arrangements, including a string orchestra.
In June 1974, there were plans for a joint appearance by Slapp Happy and Virgin label mates Henry Cow and Robert Wyatt at a free concert in Hyde Park in London, but this was cancelled at the last minute. However, on 25 June Slapp Happy recorded a Top Gear session for the BBC, enlisting the help of former or current Cow members Geoff Leigh, Fred Frith and Lindsay Cooper, plus Robert Wyatt, who contributed guest vocals and percussion to a version of Blegvad's "A Little Something" from Casablanca Moon. Credited as "Slapp Happy & Friends", this was later released in 1994 on Wyatt's compilation album, Flotsam Jetsam.
The idea to collaborate with Henry Cow eventually materialised in November 1974 when Slapp Happy asked them to provide instrumental backing, much as Faust had done on the first two albums. The resulting Desperate Straights was released under the name "Slapp Happy/Henry Cow". The success of this collaboration surprised everyone, considering how dissimilar the two bands were, and the two bands merged. The music often had a Berlin Cabaret feel about it with a taste of avant-garde jazz.
The merged group returned to the studio in early 1975 to record Henry Cow's In Praise of Learning (as "Henry Cow/Slapp Happy"). The only real contribution from Slapp Happy (besides Dagmar's singing) was the Moore/Blegvad song "War", which blended in well with the album's political aggression. But differences in approach between the two groups had come to a head in April 1975 and Anthony Moore and Peter Blegvad quit, suggesting that Henry Cow's music was too serious (and political) for their liking. Dagmar Krause, however, elected to remain with Henry Cow, who needed a vocalist. This effectively spelt the end of Slapp Happy as a band. Slapp Happy did, however, record one more single, "Johnny's Dead", without Krause, which was released in July 1975.
(pop-culture.fandom.com/wiki/Slapp_Happy)