I might be able to help but only a little. I managed a couple of trips to Main Rising in the 80's just after the link was opened up but on both occasions we dived in through Treasury and I do recall it was bit snug on one occasion. I did take a picture of the 'slug trail' out of Egnaro Aven as it was so striking. Don't know whether it looks like that now after 40 years but here it is. I included the shot of the ladder base as it shows the sludge pushed down the pitch