Kent-based artist Ralph Steadman produced some of the most ferocious, savage art of the 20th and 21st century and it was all For No Good Reason.

The legendary collaborator of American writer Hunter S Thompson, he is the man with the ink-splattered visions which decorate the pages of the likes of the seminal Fear and Loathing in Las Vegas.

This week, Sky Atlantic is screening a documentary based on Ralph’s wild life and work, called For No Good Reason.

At the opening of a free exhibition to tie in with the screening, Ralph said: “That phrase came from Hunter.

“I used to say, ‘Hunter, why are we doing this?’ He would say ‘For no good reason, Ralph’. And that’s how it started.”

Among the things Hunter S Thompson and Ralph Steadman, 78, had no good reason for doing were wild and dark assignments involving mace, gun fights with beat icon William S Burroughs and an attempt to spray paint “F*** the Pope” onto the side of an Americas Cup yacht while high on hallucinogens.

Director Charlie Paul filmed Ralph, his friends and fans – including Johnny Depp and Richard E Grant, Terry Gilliam – for 15 years before weaving it into a fascinating documentary with animation and music created specially by artists such as Slash.

Ralph said: “At one time I thought he was a vowl.

“There’s a lot of animals in our garden. He didn’t have one camera, he had 64 cameras. He was bringing a different one every time.

“He was polluting the valley with cameras. They were all over the place. I couldn’t even...go for a wee.”

In the documentary, Ralph said: “Cartooning to me meant more than just funny pictures. I needed to apply it as a weapon. I really thought if I ever learnt to draw properly I would try to change the world for the better.”

For years, Ralph had stopped drawing his scathing cartoons of politicians.

But his strength of feeling has not receded – he calls David Cameron ‘David Camshaft’ – and this week he created a UKIP-inspired cover for The Big Issue.

He said: “Cameron, he’s a bubble face. And so is George Osbourne.

“I’ve done the cover of The Big Issue and I’ve done Another Fine Mess and it is a fine mess.

“I have done a drawing of Farage called Gutter cartoon. It is narrow and it is meant to go in the gutter (of the paper).

“That’s the place it should be.

“That’s the reality I try to bring to what I do. It’s a play on words but also means something if you can actually use the magazine as a plaything and a tool of creativity. That’s how you should do all this.”

For No Good Reason is on Sky Atlantic at 9pm on Wednesday (June 4) as part of its three-part Documentary Films series. A retrospective of his work is on display at The White Space Gallery in London until Wednesday and is free.