In reply to Ian Archer:
Ian, I called in today and the situation is this.
The boulder is not fenced off but there is a sign saying "Please Do Not Climb". Nevertheless I asked about climbing on it and was given the following contact details.
Cranmer Webb - Operations Manager
Compton Verney
Warwickshire
CV35 9HZ
Tel: 01926 645500
Email: cvht@comptonverney.org.uk
I wasn't given a definative yes or no but I guess granting permission would involve signing some kind of waver should you come to grief whilst climbing on it.
I hadn't been before and to be honest I expected more. It's relatively small and as BennyBoy said there are bits breaking off it. When I looked today there were two such fresh looking pieces laying on the ground under the over hanging section.
Hope that helps.