When a group of young YouTubers organised a gathering and 3,000 people turned up, it was evident they’d hit on something big. Little over five years later, that ‘gathering’ has evolved into Summer In The City, the UK’s largest YouTube convention, now held in the 20,000-capacity London ExCeL.

The burgeoning scene did not go unnoticed by Kilimanjaro Live’s Mark Walker, who made his name working with artists such as Katy Perry, Parkway Drive, You Me At Six and Alkaline Trio, ...