Like you, I spotted this from the A303, and pulled into the layby to take a look. It's marked on the OS map, so I did some checking when I got home and found the linked web site.

I must admit, I didn't try too hard to gain access as I was on my way to Cornwall at the time. The 'Private Land Keep Out' sign was enough to send me scuttling back to the car to continue the journey.

Love to know what's visible inside if you do manage to 'arrange' access...