The easiest way of finding Private beach is as foss says travel south from Cresswell for a few hundred yards, watch on the right hand side for a caravan park there will be a car park on the left hand side of the road just opposite the caravans, park here and go down the steps, Private beach is slightly to the left as you go down onto the beach.
Not sure what the car park is like for safety, I have fished here a couple of times and had no problems and I've never heard of any problems but that's not to say your car would be 100% safe, then again there's no such thing now.
The advantage of this mark is you can still fish it with a big sea on as the outer carrs take the sting out of the sea, mind you I've never tried it on a big sea, that's just what I've heard. It can be a cracking mark though if the conditions are right.