The process of making an album can be mundane and repetitive, but it is redeemed by friendship, collaboration and the service of a higher purpose. That is one of the messages of Polly Jean Harvey's demystifying yet mischievous public recording installation at Somerset House.

On arrival, minimal directions create a sense of being invited to a poorly organised conference. But somehow that makes the whole thing seem more private and even illicit in an age where artists are obsessed with the privacy and integrity of their recordings pre-release. The only overt signs that the event is curated (by contemporary art collective Artangel) are the framed lyric sheets and posters in the holding area. Harvey will have a poetry collection published later this year and admits in the programme "these days I make the words work on a page first. Lyrics have become extremely important to me".

The lyric sheets, on which it is possible to make out a reference to the UN High Commission for Refugees, indicate that the "album in progress" has been in progress long before the sessions themselves. There are plenty of hints that the issues of British military adventurism explored on 2011's Let England Shake will widen to a more global perspective on the finished record.

We are escorted down several flights to a beige room buried in the basement of the much grander surroundings of Somerset House. The programme says much about the importance of place but the recording room could be anywhere. From our narrow corridor, we watch and hear the musicians enclosed behind one-way glass, surrounded by guitars, saxophones, drum paraphernalia, monitors and amps. A coat of arms saying "PJ Harvey" emblazoned on the wall and the kit's bass drum is the only sign of who we're here to see. A white progress sheet of songs and chords is pinned to the wall.

Our group's 45 minutes has its own little narrative: the rehearsing and rehearsing of a few bars, a chord pattern, probably from a verse of one of the songs. Two electric guitars and two acoustic guitars, mediated by genial producer Flood, who sits on a white sofa watching and commenting, casual but authoritative. The three male guitarists mutter and smirk at each other. There is much communication with eyes. They try a few different approaches to the rhythm, Harvey swinging her arms as if summoning a sea shanty, before they decide to overdub the electric guitars as recording the section live was proving a challenge. Harvey doesn't sing live but we hear her guide vocals from the tape. Plans are made for Tuesday's session.

Finally, we are told our slot is over and the sound from the room is cut. The band continues and we leave them, now muted, as if they have always been and always will be in a recording session. The few bars linger in our heads. It's almost exhausting.

Fears that our 45 minutes would consist of watching people just hanging around are fortunately unfounded. Above all, I feel as if we've witnessed an unspoken understanding and camaraderie between the long-term friends and collaborators. When we buy music, there is a feeling of inevitability - that the songs were always meant to sound like that. Recording In Progress reveals all the tiny decisions and uncertainties that lie behind every fragment of the finished product - the forks in the road. There has been much speculation about how the musicians would behave differently or even perform differently when under observation but we do not see much evidence of self-consciousness creeping into the performances apart from a wry aside from Flood that "that was one of the most exciting moments in recording history!" Who knows if the recipients of a different 45 minutes will feel the same.