The wonderful old La Rocco Tower, the iconic centerpiece of St.Ouen's Bay - in the St.Brelade's section of the bay by the looks of the map in the phone book. One Sunday some weeks ago it was one of the properties featured on the heritage sites open day...
You would have to bring your own ladder to get through up to the roof level. It looks like the Germans reinforced it somewhat.
.. and an empty level after the crowds had left - the fireplace is on the opposite side on this floor, in a Corbiere direction.
Many thanks to my roving reporter, it gives me a really good idea what it's like in there, after years of wondering!