Like I said in my last post as the area is used so much by other's I think it's a genuine mistake on their part as one of them get's to farm on it and it's probably assumption that it's ok to take your vehicle up there. Personally I've only ever seen one quad on the top of Moel Tryfan and he was a local farmer's lad messing about. The now defunct quarry below it on the other hand is VERY busy with various 4x4's, quad's and scramblers.