I spent absolutely ages trying to find this waterfall! Happily, it was worth the effort. The Bolton Abbey estate covers a large area with varied scenery. There is a walk - known as the Valley of Desolation - which I'd never done before. It follows a little stream (Posforth Gill) up the valley and eventually on to Barden Fell and the hill known as Simon's Seat. The Gill flows over this lip in a rather attractive waterfall. Whatever desolation there once was (caused apparently by a huge storm in 1826) has long since been repaired by Mother Nature.
To find the falls, you had to take a steep path down to the left off the main footpath but I missed the turn and went on upwards, nearly making it onto Simon's Seat, before I realised my error. Never mind, it was a good walk and good to get the exercise. I do a lot of walking but mostly along fairly flat paths (like the canal towpath) so striding up a hill for a change is good for my heart and lungs!