I think it was a small frame marquee at the back (I recall sitting in it - and it was for overflow from the bar, not provided as an entertainment venue - the performances were taking place indoors - although there might well have been 'impromptu' singing outside), not a gazebo - there might have been a gazebo but that was just providing shelter for staff (different matter). Presumably the marquee would have been erected by a professional company(?) and all the required risk assessments, method statements etc provided.

I'm saying this with tongue slightly in cheek, partly just to show that there ain't nothing simple in this, once you start looking at doing it.

Of course, it would also now require a TEN.