Oki Doki, I've found some more info online - hard to find... Welsh water seem to own the land / track, but have leased it to the Elan Valley Trust.
This bit is cut and paste word for word from a 2004 Land Rover forum... it refers to the section from the Ford / Bridge, to the actual tarmac road section, marked as "track" on the OS maps.
"I know this bit of track very well. It is indeed owned by by the Elan vally trust. It has also been known for people to get heavy fines if cought!. This whole section was bought up at a ROW meeting not so long ago. From what I gatherd it was declasified to stop a though route. The Elan vally trust have alot of wait behind the name, They were the main voice behind Monks Trod being TRO'd They are now pushing for full closure. If you are coming from the north end there isnt any signs saying it is privite untill you have cross the bridge. Also have been told the farmer will wave and be very nice but soon as you have gone he is on the phone. Personaly I would keep clear from this track as it only set to upset the Elan Trust more!"
Also another land Rover Forum thread on that stretch.
http://forum.lro.com/viewtopic.php?f=16&t=22532&start=10