They usually confined their activities to “dilapidated buildings, walls made of corrugated iron, smashed-up cars, and nothing on people’s property”. But, he admits, they didn’t always stick to their own rules. Once, they sprayed their names on a garage door. “And a few weeks later, George Melly wrote a piece in the Guardian or the Times saying, ‘I came out of my garage recently to find that people had sprayed graffiti on it. If I ever catch that Mr B, Kix and Columbo, I’m going to kick their arses.’

“So that was our claim to fame,” says Thompson today, his graffiti career long ended, his real claim to fame being his subsequent career as the frequently airbound saxophonist in Madness, formed by his pseudonymous friend Mr B, the band’s keyboard player, Mike Barson. That would have been the end of the story, save for the fact that, not long afterwards, Thompson discovered that photographs of his nickname, daubed on a wrecked car and a wall in Kentish Town, had appeared in a book called The Writing on the Wall.

The work of Time Out photographer Roger Perry (who died in 1991) and designer Pearce Marchbank, the book documented the graffiti of mid-70s London and came with an introduction proudly claiming the slogans it contained were “a protest on the part of the individual against the mass and its masters” and “proof that human beings still exist in a world of supermarkets, office blocks, processed chickens, VAT forms, computers, ECT”. It also decried those who objected to graffiti as “offended to the bottom of their 19th-century souls by the messiness of the individual”. To Thompson’s considerable surprise, the introduction’s author was George Melly. “How about that?” he laughs. “One minute he wants to kick my arse, next he’s praising me!”

Melly’s volte-face is not the only remarkable thing about The Writing on the Wall, about to be republished after being out of print for decades. Perry’s photos offer a vivid snapshot of British culture in the mid-70s, between the final curdling of the hippy counterculture and the arrival of punk.

“In 1975, graffiti was a shorthand way of accessing the mood of the time,” says writer Jon Savage, who mentioned The Writing on the Wall in his 1992 history of punk, England’s Dreaming. “In the 60s and even the early 70s, music had reflected the environment and how people felt, how people thought about things – and that was almost gone. Pop wasn’t doing its job, it wasn’t the teenage news. Graffiti was like a secret code, the voice of the underdog. It was people telling you things you couldn’t read in mainstream media and wouldn’t necessarily think about. You’d get jokes, stoner and outcast humour, with serious points. It was another kind of language.”

There are political slogans relating to dimly remembered campaigns: the trials of the Angry Brigade members, known as the Stoke Newington 8, and of the Shrewsbury pickets, imprisoned in the wake of the 1972 building worker’s strike, whose number included actor Ricky Tomlinson. There are quotations from Shelley and Blake, and slogans from members of King Mob, a British splinter group from the Situationist International, whose associates included Malcolm McLaren and designer Jamie Reid. King Mob were responsible for perhaps the best-known graffiti in the book, sprayed along the tube line between Ladbroke Grove and Westbourne Park so that passengers could see it: “SAME THING DAY AFTER DAY – TUBE – WORK – DINNER – WORK – TUBE – ARMCHAIR – TV – SLEEP – TUBE – WORK – HOW MUCH MORE CAN YOU TAKE – ONE IN TEN GO MAD – ONE IN FIVE CRACKS UP.” Pink Floyd’s Roger Waters subsequently claimed this inspired the lyrics of Time, from Dark Side of the Moon.

Some of the graffiti in the book is absurdly funny or strangely wistful: “Remember The Truth Dentist”, “Cats Like Plain Crisps”, “We Teach All Hearts To Break” on the wall of a school. Some isn’t: “Go And See Stardust With David Essex It’s Really Great”. Exactly who was responsible for what is unclear; Thompson aside, the authors usually chose to remain anonymous. But a lot of the slogans  were the work of poet and dramatist Heathcote Williams, then involved with an anarchist group called The Albion Free State. “Why graffiti?” he says. “A lark. The anonymity is appealing. It’s not signature art. It’s just a thought, which can either be the view of a lone crank or be the view of thousands. It’s up to the reader to decide. They bubble up in an engagingly mysterious way. You think, ‘What’s that doing there? Who on earth wrote that?’”

The first graffiti Williams ever saw was as a child in Reading Museum. “A builder had written on a Roman tile with his finger in the unfired clay, ‘Satis’ – ie, ‘That’s it, I’ve had enough, eff this for a game of soldiers.’ And then he’d presumably quit work. But there’s something vulgar about taking any credit for them. They appeared anonymously and perhaps should remain so.”

There were other, more prosaic reasons for the proliferation of political graffiti in early-70s London. “It’s such a bloody cheap thrill,” says activist Mike Lesser, who worked alongside Williams. “You just buy a spraycan for almost nothing, you write something, you choose a good position, and it takes a bit of time for it to get washed off. You could put up posters, but that takes four or five crews, two or three weeks, and you have to have a very heavy-duty, well-organised outfit.”

Perry’s camera also captured a now lost London. “The photographs are absolutely beautiful and the way things looked back then seems incredible,” says Paris 1974, a latterday graffiti artist whose designs adorned the cover of Coldplay’s Mylo Xyloto album. He’s long been a fan of Perry’s book. “It wasn’t even that long ago, but everything still looks so postwar and battered.”

Most of the graffiti in the book was found in and around a strikingly ungentrified Notting Hill. “I used to go down to Basing Street a lot because Island Records had their studios there,” says Pearce Marchbank. “It was really rough around there at night. You’d come out of the studio at 4am and it felt like people were sharpening their knives on the corners of buildings, that kind of thing. Now it’s David Cameron, isn’t it? Shops that sell scented candles.”

Savage, who took his own photos of the area in January 1977 for his punk fanzine England’s Outrage, says: “It was the gap between slum clearance and new build. People had cleared all these Victorian houses, thought of as slum housing, but they didn’t have the money to actually build new houses, so there was a gap. There were a lot of squats, which was a way in which young people who didn’t have much money – beatniks and bohemians – could live near the centre of town. It sounds really wanky, but it was almost as if those forgotten and decrepit areas held a kind of truth.”

Looking at the new edition of The Writing on the Wall – the original book augmented by further photos and with new introductions by Bill Drummond and George Stewart-Lockhart, an art student and graffiti fan who led the Kickstarter campaign to get it republished – it’s hard not to feel that the kind of graffiti it documents has largely died out. Forty years on, there’s plenty out there, but it tends to be more straightforward. Interested largely in writing his nickname, Thompson turned out to be an unwitting pioneer of the “tags” that now proliferate, imported from New York hip-hop culture (ironically, as Paris 1974 notes, the first large piece of New York-style hip hop graffiti in Britain appeared in Notting Hill in 1981, the work of American artist Futura 2000, who was touring the UK with The Clash).

Although you don’t see the same profusion of surreal political slogans today, perhaps the stencil works of Banksy et al are their descendants. One theory is that The Writing on the Wall captured a more politically engaged era when, as Marchbank puts it, “politics was fashionable”. Savage thinks the shift from slogans to names may reflect a shift in British society: “The 70s was the end of the collective era. You then had 30 years of almost unrestrained individualism, which was the Thatcherite, New Right revolution really – there’s no such thing as society, there isn’t a collective, there’s only an individual.”

Another theory is that methods of communication have changed. If you had a political slogan you wanted to get across in 2015, you’d probably stick it on social media or try to turn it into a hashtag campaign rather than spray it on a wall. Your potential audience is vastly increased but, as Paris 1974 points out, whether people would take as much notice is a moot point.

“Your phone or your computer – you can turn it off, can’t you?” he says. “But something that’s actually in the physical world is more lasting. It becomes part of the tapestry of the city. Thousands of people have no choice but to see it – it gets engrained in collective psyches. I’m really pleased this book’s coming out again because this type of graffiti is important. The other type, which I’ve been part of for a long time, gets good press and bad press – and I don’t really know, as an artform, where it’s going any more. A lot of walls don’t really make any sense. They’ve just become a commodity, under Perspex. But this, this is back to the real business.”

• The Writing On The Wall, published by Plain Crisp Books,price £25. Details: rogerperrybook.com