On a later visit I have found another shell which I think is possibly the same type. I reckon that the textured surface you see in images 3 + 4 has been eroded away to an extent resulting in what you see in image 1 + 2.
On the underside of image 4. you can see a bare patch a bit like the one in image.1
Now I am thinking that this is some sort of life form that has covered the shell the way baracles and limpets do and not a shell in it's natural form? (does this make sense