Answer by taking a quote from the following article:

John Glascock (2 May 1951 - 17 November 1979) was the bass guitarist, backing vocalist, and occasional lead vocalist for the progressive rock band Carmen. He was also the bass guitarist and backing vocalist for the progressive rock band Jethro Tull from December 1975 until August 1979. He died in 1979, at the age of 28, as a result of a congenital heart valve defect, which was worsened by an infection caused by an abscessed tooth. In a September 1978 'Guitar Player' interview, Ritchie Blackmore had said "...

In 1973 he joined Carmen. With them, John Glascock played bass guitar, sang backup vocals, occasionally played synthesizers, and even sang lead vocals on a few songs. These songs include "The City", "Dancing On a Cold Wind" (sharing lead vocals with David Allen), and "The Horseman" (sharing vocal duties with all of Carmen), all from the album Dancing on a Cold Wind (1975). He also sang on the track "High Time" from the album The Gypsies (1976).  Glascock's basslines in Carmen were often technical and unusual. They were key to Carmen's unique sound. Unlike his work in Jethro Tull, Glascock utilized effects in his sound while with Carmen, like a fuzz pedal in the song "Viva Mi Sevilla". Glascock had only one writing credit on the first album recorded by Carmen, Fandangos in Space (1974), the short track "Retirando". He shared this writing credit with the rest of the band.  His writing expanded greatly by the time of the second album, Dancing on a Cold Wind (1975). On this album he had a writing credit for three songs, "Viva Mi Sevilla" (which he shares with the rest of the band), "Purple Flowers" (which he shares with Roberto Amaral), and "Remembrances" (which he shares with the rest of the band).  His writing had reached a climax by the time of Carmen's third and final album, The Gypsies (1976). On this album he wrote "High Time" by himself, and sang lead vocals on it. He and the rest of the band also share a writing credit on the instrumental track "Margarita".

how did the carmen story begin?
In 1973 he joined Carmen.