Back in 1985 or 86 we had permission to be in Dinorwic to recover some of the wagons and other items for preservation. We even had the keys to the place. We clooected stuff from all over the quarry either almost directly by road or by bring items down C4 and C5 inclines. We gathered all of it together on a flat area near bottom right of quarry (possibly C1T level, I forget now). On one occasion we parked a car there (Vauxhaul Cavalier Mk 1) having parked, with handbrake applied, we opened the driver and front passenger car doors, at that point the car started moving forward because the wind was so strong it was over coming the hand brake! The driver had to put on handbrake extra hard AND put it in gear. It was on perfectly level ground.
We had a good time those few times. Bombing up and down the quarry roads in Land Rovers etc.
A photograph and article on our exploits appeared in the CEGB staff newspaper.