Red Humphrey the Highwayman?

Apart from its name, Nant Siambar Wmffre is an otherwise insignificant stream that drops down from Tarren y Gesail to join the Dulas near Pantperthog. The 1891 map gives the spelling as Nant Siambr Wmphre, and shows its steep little valley as having the name Cwm Siambr Wmphre. I thought it odd that an obscure cleft in the hillside at the most only 2 miles long should warrant its own name, and that there must be some story attached to it. After much searching, I came across a mention of it in a Ward Lock tourist guide to Wales from the 1970s (why did they pick on this obscure feature?), referring to the "Chamber of Hwmffra Goch" as being "a cave behind a waterfall which long served as a highwayman's hide" near Llwyngwern. The same publication also makes mention of the nearby "Gelligen Stone Chests" which are "excavations formed by falling water". I'm trying to find out more.