Herne Hill Saturday Training

. They picking up speed in the excitement down from the bank then start to panic at the entry therefore massive braking.

You are correct about the effect but it's not so much caused by the above.

You should have been told how to ride a banked track at a constant speed, ie. pressing on into the turns and soft pedalling out of them. There's even a diagram on this forum somewhere.
It's simply riders failing to control their speed, and instead letting the track geometry dictate it. This might be fine for the first two riders in the group, but as you quite rightly say, this is magnified by a massive amount by the time you get to the back of the group. It's also exacerbated by riders chasing to fill up "gaps" (why do you need to be 2" away in a basic training session?) and riders kicking back (leg braking) when they approach the rider in front.

There are no circumstances in which 70 novice riders should be on the track in the same bunch, or be expected to deal with it. It can take a fair bit of track time before a rider is capable of applying the protocol effectively.