Answer by taking a quote from the following article:

Van der Graaf Generator are an English progressive rock band, formed in 1967 in Manchester by singer-songwriters Peter Hammill and Chris Judge Smith and the first act signed by Charisma Records. They did not experience much commercial success in the UK, but became popular in Italy during the 1970s. In 2005 the band reformed. The band formed at Manchester University, but settled in London where they signed with Charisma.

Though the group have generally been commercially unsuccessful outside of early 1970s Italy, they have inspired several notable musicians, including Rush, John Lydon, Marc Almond, Graham Coxon, Luca Prodan, Mark E. Smith, John Frusciante, Bruce Dickinson and Julian Cope. Dickinson, who has been a fan of the band since he saw them at Oundle School aged 13, said that Hammill was one of his childhood heroes. Coxon is particularly fond of "House with No Door" from H to He (1970), saying the track is "extremely beautiful, with Jackson's truly lovely sax-and-flute instrumental section." Almond recalled hearing "Killer" for the first time saying, "I'd never heard anything like it before. It wasn't just Peter's snarling operatic vocal, it was the mix of instruments ... I became an instant fan." Mentioning their reputation as something of an acquired taste, Lydon said, "There's a few Van Der Graaf things I like, but I'm not going to recommend anything to anyone. It might not be for them. Music doesn't come with a set of guidelines." Although generally categorised as a progressive rock group, Cope is keen to distance the band from that movement, stating "Their music was like some Brechtian bar band - the opposite of prog rock, really". Nevertheless, the band have been acknowledged as an influence on the neo-progressive rock subgenre which emerged in the 1980s and featured Marillion as its most successful band. Marillion singer Fish thought highly of Hammill and invited him to be the support on the band's early tours. The Canadian new wave band Men Without Hats have a song titled "The Van der Graaf Generator Blues" on their 1991 album Sideways.

When was the group popular?
Though the group have generally been commercially unsuccessful outside of early 1970s Italy,