Hi Colin - The last people to work Silverband was Waitings Minerals in the late 1990's, after Tommy Craggs opencast the area in the 1980's. I would expect Waitings Minerals will have had to take on the job of reinstatement and responsibilty of the clean up and maintenance of the site and perhaps should have the key to the barrier. There address was Glebe House, Cliburn, Penrith, sorry don't know the number.
Permission to use the private road that goes up from above the Silverband Sheep Fold Gate to the turn off for the Barrier road that may be harder to get, as this road is operated for and by the CAA on the top of Dunfell.
John L