Who are or what is Massive Attack? At times it seems to be less a band or a group than a kind of movement, with a large cast of associate and affiliate members and an ever-changing line-up of musicians, producers, DJs and vocalists. If there is a central system of control, it probably revolves around Robert Del Naja, who has appeared on each of the four Massive Attack studio albums and has overseen the selections for Collected, a new greatest-hits compilation that includes previously unreleased material. And if there is a base, it is Bristol, even though many of those who are in or have worked with Massive Attack no longer live in that vibrant, multiracial city.

Yet through all the changes in personnel, and for all the various singers who have worked with the Massives, as they are more colloquially known, the sound has remained unmistakably the same. Their first album, Blue Lines (1991), established a template, and it remains one of the most influential records of the past 20 years. Its fusion of electronics with rap, dub, hip-hop and reggae vocals, its bleak lyricism and evocation of urban alienation and paranoia, its use of sampling, and its dense, multi-layered sound have been widely imitated but never bettered. As the founding member Grant Marshall (aka Daddy G) said, here was “dance music for the head rather than for the feet”.

On subsequent albums, the mood and the sound fractured and darkened; everything seemed to slow down, as personal relations became strained and the Massives began to experiment less with dance rhythms than with more somnambulant electronic backbeats, inviting the likes of Tracey Thorn (of Everything But the Girl) and Liz Fraser (of the Cocteau Twins) to sing on selected tracks for the albums Protection (1995) and Mezzanine (1998). The title track of Protection, featuring the soulful Thorn, with its mournful, trance-like electronic underbeat, is, for me, the best of Massive Attack. Ambient dub, trip-hop - whatever you call it, this was late-night, post-rave, spliff-slowed mood music with its own peculiarly beguiling atmosphere of confusion and loss.

Massive Attack emerged in the early 1990s from a Bristol-based group of DJs and musicians called the Wild Bunch sound system. As a group, the Massives were multiracial, like Bristol itself, with its deep history and problematic connections to the slave trade, its inner-city deprivation and cultural diversity. The group was interested in a range of musical styles, from dub to jazz to electronica, and in how they could be combined or collapsed into each other to create a new kind of contemplative urban dance music. The early Massive Attack was, broadly, a trio: Del Naja, Grant Marshall and Andrew Vowles. They were supported by the producer Nellee Hooper of Soul II Soul; the reggae artist Horace Andy; and the rapper Tricky, who went on to release one of the best and most disturbing albums of the Nineties, Maxinquaye (1995). In addition, there were the soul singer Shara Nelson and the multi-instrumentalist/sound engineer Geoff Barrow. With the singer Beth Gibbons, Barrow later set up his own group, Portishead, which took its name from the small town just outside Bristol. A large and varied cast indeed.

A few years ago there was a series on BBC2 called Rock Family Trees, which traced the connections between groups of musicians and showed how each had influenced the other, and who had worked with whom and when. It would be possible to make a similar programme about Massive Attack; it would be quite long, and the resulting family tree would have many branches and offshoots.

Take, for example, Tricky, who was such an influence, for his songwriting as well as his inimitable, flat-vowelled rapping, on both Blue Lines and Protection. Later, once he had stopped working with the Massives, Tricky collaborated on Maxinquaye with, among others, a young Alison Goldfrapp - well known today as the eccentrically flamboyant singer/songwriter of the chart band Goldfrapp - and the then teenaged Martina Topley-Bird, with whom he had a child. After their separation, the lovely Topley-Bird released her own album, the Mercury Award-shortlisted Quixotic (2003).

The connections do not end there. There are songs on Maxinquaye that also appear, in different forms and under dif- ferent titles, on Protection; “Overcome” (Maxinquaye) and “Karmacoma” (Protection) are essentially the same song. Another song from that album, “Hell Is Around the Corner”, has the same backing track, if not the same lyrics, as “Glory Box” from Portishead’s first album, Dummy (1994). And so the samplings and borrowings of the Bristol scene go on and on.

Of late the Massive Attack sound has become all-pervasive, their songs and instrumentals being used in films and TV programmes as theme music and soundtracks. But we have had nothing substantial from them since the unremarkable 100th Window, their fourth album, released in February 2003.

There remains something furtive and unaccountable about Massive Attack. Why, for instance, does no one beyond Del Naja stay for long within the movement? Perhaps Del Naja’s personality is simply too oppressive and controlling. Or perhaps, as it was for Tricky and Barrow, the desire to achieve something in your own right is too powerful.

Whatever is going on inside the group - and one reads often of rancour and disputes - this much is true: the musicians, composers and collaborators working under the name of Massive Attack, formed by the rebellious DIY attitude of the rave scene and always seeking new ways to express the reality of the tense urban landscape through which they move or once moved, have succeeded in writing some of the most influential and seductive electronic music of modern times. In so doing, they have offered their own idiosyncratic definition of Britishness: multiracial, heterogeneous, fractious, often violent, and just about holding together.