An all-day celebration of classic rock for true rock fans is the Stone Free blueprint. It’s also about long-anticipated, nostalgic returns. In its inaugural year, among other highlights, Stone Free Festival brought an exclusive that hadn’t been witnessed since 1975 with Rick Wakeman’s performance of ‘The Myths And Legends Of King Arthur And The Knights Of The Round Table’. This year guitar maestro Ritchie Blackmore brought Rainbow to London for the first time in nearly 25 years.

But there was a lot for the crowd to savour before the main act at the classic rock all-dayer. Pop-up stages were, well, popular. Near The O2 main entrance was the Orange Amplifiers stage which provided an all-day soundtrack to the nearby vinyl fair. Music fans listened while they browsed and shopped.

On that stage, Buck & Evans brought the rock and soul, Tequila Mockingbyrd and Massive delivered energetic riffs and solos and Evil Blizzard provided pig masks and downright weirdness. Outside was another small stage set in a very sunny beer garden conveniently located next to a craft beer trailer.

Meanwhile, entering Indigo in the O2 (the equivalent of the second stage) in the early afternoon meant entering The Crazy World Of Arthur Brown. The eponymous frontman is a long frame of strangely languid, colourful energy, conducting his band through an incredibly tight set that trembled with odd costumes, even odder dance moves and bizarre, walk-on characters.

Up next were Scottish hard rockers GUN who performed a hits set with aplomb, before The Answer took to the stage and belted out a series of songs filled with no-nonsense, bluesy riffs. Then Indigo was stretched to its limits as fans flocked to see Blue Oyster Cult. Celebrating their 45th anniversary, BOC were performing their first album in its entirety, interspersed with a number of their greatest hits and yes that song. The band seem to be almost more popular now than at any other point in their career.

Away from the stages, festival-goers could find quieter entertainment in the Speak Easy Lounge, which featured a comedy and spoken word line-up and in Cineworld, which had a special screening of The Wickerman.

Support for the headline act came in the form of Sweet who the most enthusiastic band of the day and looked to be having a whale of a time in the arena. Squint and (okay you would need to really, really squint and maybe look in the other direction), but squint hard enough and you could almost imagine it was the original teenage troubadours belting out the likes of ‘Teenage Rampage’, ‘Wig Wam Bam’ and ‘Ballroom Blitz’.

The main event was one for classic rock connoisseurs. Exquisite musicianship was illuminated by some classics that hadn’t been heard live in the capital since 1994. These included Rainbow’s most-famous songs, a smattering of Deep Purple favourites and a version of ‘Since You’ve Been Gone’ that Blackmore brought out its writer Russ Ballard to play on, creating another piece of classic rock history in the process.