Martin,
We stayed at CC Plymouth Sound in July. As has been said, it's a no-toilets/showers site, but that's no problem. The views are lovely, especially if you manage to get on one of the front pitches. Most of these are steeply sloping, but the warden is happy to position you with his tractor if required. He and his wife are very helpful and pleasant.
Mention has been made of access through the lanes being "a bit tight". I would have said very tight in some parts, and we got stuck for a bit in a very awkward place on the way back (and ours is only a short van, though 7'2" wide). Still, people had obviously had got in and out with very large vans and at least one full-sized American RV!
Some nice walks around, and an easy solo drive through the lanes down to Mountbatten where you can park free all day and catch the foot ferry back and forth to the Barbican for