Thanks for the heads up on this, though sadly will be able to attend. I have been to Crossness, many years ago, and would heartily back up the recommendation. It is a grade 1 listed building - a true cathedral to the industrial age, prosaic yet beautiful and a visible reminder of every Londoner's debt to Bazalgette.

I didn't see the engines operating though as they were still being restored when I went. Thinking about the mammoth proportions of the pistons alone, I would expect this to be quite a spectacle.