Fretwork,
I am sorry, I do not know of the history of a slippery dip down at the pool. I have only been up here since 2008 and I have never been to the local swimming baths apart from two days ago which I drove to; to check for you, if a slippery dip still existed. (What a nice peaceful hidden area of Blackheath!!). I didn't see a slippery dip. Just the changing blocks and the swimming pools. :-)