These sessions are a great idea, which may take a while to get it right. I believe that the pace is right, however what is happening is that because the group was still too big for the pace we were going, riders are staying on the front for too long.
If chains were no bigger than 4-5 riders, restrict the speed to an agreed pace, make sure there is 1 strong rider in each group to marshal. This would result in everyone doing more work yet maintaining a safe speed?
Just a thought? (Can't make it next week due to shift work)