I usually park in Stanshope, and then follow the path into Hall Dale through a couple of fields. Once over the last wall, I turn left and go up the steep slope formed by the side of the valley, right to the top by the wall. Now follow the wall along (passing two gates) to where a short barbed wire fence leaves the wall, and goes diagonally towards you, down the hill. Walk between the end of this and a small rocky knoll, heading down the hill. The cave is pretty much directly beneath this knoll - just where the bushes and trees start.When at the cave, you can get down to the main path again by going straight down the steep scree bank (This might form an easier approach when you know exactly where to go up) - but I prefer to go back the same way.
If you stand on the rocky knoll and look directly at Illam rock I'd say the tube is at about 2 or 3 o'clock, maybe 50m away down the hill side.