As it happens I was walking there yesterday to look at the waterfalls and to see how the scheme at the old Gwynfynydd mill was progressing.
A new intake weir has been constructed but not much other progress is visible (presumably they are waiting approval of their revised planning application). There’s a turbine draft tube sitting on top of a container in the work compound at the old mill site. The area in front of the Gwynfynnydd adit seems to be in use as a temporary storage area as there's a volute casing fabrication for a turbine (painted the same colour as the draft tube), a butterfly valve and other items that would appear to be associated with the project.
Lengths of high voltage cable have been trenched-in (but not jointed) along the access road above Ferndale and there’s a new pole-mounted transformer on the approach side (as you come from Ferndale) of the bridge over the Afon Gain. The bridge currently has had a new (temporary?) bridge erected over the top of it.
The revised planning application (NP5/60/133C) is to modify the approved scheme (NP5/60/133), which had one 560 mm diameter pipe, to have two 630 mm diameter pipes (and, from the application plans, an additional turbine).
http://www.eryri-npa.gov.uk/planning Dave