Is this where the steel footbridge used to span the gap, and where the floor of a tunnel had fallen through leaving a hole into the cave below?

Think so if this was a later bridge in the same place. A few years ago this was the only hole I found in the cave roof.

The bridge as I recall was painted steel in 57/58, and yes the hole looks right, amazing what you remember. Someone had attached a gas mask to a head shaped boulder and thrown it into the water, when you crossed the bridge you could look down and see the two goggle eyes looking back up at you.