Street art with a political purpose has a much shorter shelf life than those evocative old shop signs and advertising ghost signs which linger on London's gable walls. Much of the political sloganising is done quickly and transgressively and is lucky to last for more than a few weeks.

In our patch, we have a rare survivor – a mural with a message which dates back more than forty years.

Highgate is fortunate in sporting the Smiling Sun of Dartmouth Park Hill, a splendid emblem of the early green movement.

It's on the stretch of the road just south of the Whittington Hospital, at the junction with Hargrave Park. And although this sun is slowly setting – or, more precisely, fading with the passage of the years – it's worth seeking out.

I must have first come across this cheerful piece of public art not all that long after it was painted. I now live just a few minutes walk away. On my blog quite a few years back, I posted about this Smiling Sun and asked if anyone knew its back story.

First of all, I got a message saying that the mural was painted by Kelvin "the mushroom maniac" when he was squatting in that house in the Seventies.

Quite some time later, I heard from Kelvin himself who had moved to a disused chapel in Cornwall.

He sent me a copy of Poisoned Power, the influential polemic against nuclear energy prompted by the Three Mile Island partial meltdown in the US. And he provided an account of how the Dartmouth Park Hill sun was born back in the days when he was "a young headstrong hothead".

"It must have been (incredibly) 1976," he wrote. "Homeless and living on £5 a week, we heard of the incredible squatting movement in that area at the time – and with a massive sigh of relief we moved into that house!"

He was prompted to get his paint brush out on his return from a bruising demonstration outside the Atomic Weapons Establishment research facility at Aldermaston in Berkshire – and influenced too, Kelvin confesses, by some mushrooms.

"Having come across a few tins of old paint while on another blindingly enlightening anti-nuclear trip," he recalls, "I was suddenly seized with the absolute necessity to do something about it then and there. So, in the middle of the night, much to the misgivings of my long-suffering wife, and convinced I'd be getting busted for it in the morning (if not while doing it), I grabbed a ladder from a building site opposite and dashed it off."

"If I'd have known that it was to last half a lifetime I may have just taken a little more care over it. I was in such a hurry I remember I nearly fell to my probable death in the process!"

Ham & High: The Smiling Sun of Dartmouth Park Hill - photo: Andrew WhiteheadThe Smiling Sun of Dartmouth Park Hill - photo: Andrew Whitehead (Image: Andrew Whitehead)

Truth be told, Kelvin's Smiling Sun doesn't look in the least slapdash. Indeed, it's hard to believe that it was painted in the dark, on the spur of the moment, by someone under the influence of more than environmental concern. The circle is round, the lettering is clear and the design is perfectly symmetrical. It's altogether in a different realm from the scourge of spray-painted graffiti.

Kelvin's sure of the date – he remembers that the person who first got in touch with me was then a babe in arms in that same house. And the date fits. The anti-nuclear smiling sun originated in the Danish green movement in 1975 and spread rapidly.

This seems to be one of its earliest appearances in Britain. That's reflected in the slogan "ATOMIC POWER / NO THANK YOU" a slightly primmer version of what became the wording on millions of Smiling Sun badges: "NUCLEAR POWER? NO THANKS".

Andrew Whitehead is leading a walk around Three Spectacular Ghost Signs as part of the Highgate Festival. Meet outside Crick's Corner cafe at the junction of Dartmouth Park Hill and Bickerton Road at 5pm on Tuesday, June 22 – no booking required and it's free.

Ham & High: The iconic Smiling Sun badge - photo: Andrew WhiteheadThe iconic Smiling Sun badge - photo: Andrew Whitehead (Image: Andrew Whitehead)