Go to Lyndhurst and then take the A35 to Christchurch. After 5 or 6 miles through the forest there is a left turn at the bottom of a dip. This will have a (probably small) RR10 sign directing you to take the left turn. After a very short distance there is a T-junction where you should go right, again this should be signposted. From there, stay on the main road for a mile or so and there will be signs or marshalls to direct you to the car park. You won't miss this because it is all fairly open countryside at this point.