The Roundhouse wasn’t the fully kitted-out modern venue you see today. “It had fallen into disuse,” Nick says of the former engine repair turning shed and Gilbey’s Gin storehouse.

So much has been written about that night in the years since that fact and myth have become intertwined. Regular power-outs, a guest list of Beatles and Blow-Ups, acid casualties stalking dirt-floor stalls dosed up on acid-laced sugarcubes handed out on the door. What does Floyd drummer Nick Mason recall from that night, 50 years on? “Well obviously I can’t remember a thing, so I’ll have to invent it for you,” he laughs.

Bob Dylan’s first electric tour, the Velvets’ Exploding Plastic Inevitable, the Pistols and Buzzcocks in Manchester ’76. Some gigs continue to resonate for decades after the band leaves the stage. The Pink Floyd and The Soft Machine launching radical paper the International Times at the Roundhouse on October 15th 1966 is right up there.

“There was no lighting and no power as far as I remember. We actually had to bring in a 13-amp extension lead from some building just outside. Every now and again the power would overload and all sound would be lost while someone went to reset the fuse.”

What of that supposedly starry guest list?

“It was one of those things where they expected a hundred people and a thousand turned up. Certainly [Blow-Up director] Michelangelo Antonioni was there and a number of other luminaries of the time. I think Paul was there. There’s a bit of confusion because the more likely candidate would have been John, but yes, Paul did come down.”

Organiser Barry Miles was definitely spotted handing out sugarcubes to proto-hippie ravers on the door, but all wasn’t as it seemed.

“I think almost certainly there might have been sugarcubes but they weren’t laced,” Nick says. “I never heard of anyone who tripped on a sugarcube from the Roundhouse. I think it’s the sort of thing that you’d know about!”

As Ron Howard’s recent Beatles doc Eight Days A Week shows, even when the biggest band in the world was playing enormodomes and stadiums, there wasn’t much attention paid to lighting things up. The likes of The Velvet Underground and Pink Floyd changed all that, but Nick doesn’t remember much about Joel Brown’s revolutionary ink-and-slide set-up, which saw psychedelic shifting shapes projected onto the the white-clad band and backdrop.

“Whatever we did would have been unbelievably small time,” he says. “This was our first actual real gig to more than 50 people. Apart from anything else the power coming in wouldn’t have been sufficient to do very much more than a couple of amplifiers and a very modest PA system… almost any band now would have far more.”

The event did more for the wider (counter-)culture than it did for the fledgling Pink Floyd themselves. “It was the beginning of Swinging London and alternative societies,” Nick says. “It was a great story for the press. We were only a small part of it.”

The Floyd returned to the Roundhouse several times over, but nothing quite caught the madness of that self-proclaimed All-Night Rave. “It had become a proper venue – one of the places that we played if we were on an English tour.” That first night was something so much more.

“The initial Roundhouse event was as much about the audience as it was about the band. Two years later that had all disappeared. The money was in the music. It changed fairly drastically, quite quickly, as the music became commercial.”

In the years that followed, the Roundhouse hosted everyone from The Stones and David Bowie to The Ramones and The Clash, but it fell into disuse once more in 1983. Nick’s fellow Camdenite and flying enthusiast Torquil Norman nabbed it in a £3 million ‘impulse buy’ in 1996 and roped in the drummer to help get the venue back to its former glory.

Sick of stuffy dinners, Nick instead became a Roundhouse ambassador and moved into the undercroft with some donated Abbey Road Studios gear to help young bands record.

“I really enjoy it,” he says. “Working with young musicians and young engineers I’ll find out things that I didn’t know before. And sometimes I can tell them stuff…”

Speaking of the studio, Nick rules out another Floyd album to follow 2014’s record-breaking The Endless River. “Without Rick [Wright] it would be very hard to really do anything as Pink Floyd. I certainly have no indications from David [Gilmour] that he wants to do anything else.”

And while the surviving Floyd trio playing the Roundhouse is unlikely (“It would take something a bit bigger… another Live 8”), with London venues falling around our feet it feels vital that the Roundhouse continues to thrive.

What can we do to keep this special bit of London music history alive? “The main thing is to turn up. Be an audience and go and listen to live music.” We’ll see you down the front.