Thanks for the replies, folks.
I'm not as knowledgeable about DS as you lot but, when I see a phrase like "27.07.77 Charlie Gillett Session", I take it to mean a session specifically for Charlie Gillett's radio show, either recorded on or broadcast, on 27 July 1977. I had always assumed that DS had recorded these demos for sending to both radio shows and record companies, in the hope that they would lead to a recording contract. Did DS, having recorded the demos, simply make one copy and send that copy to Charlie Gillett?
As for Amazon, you are right. Amazon UK lists it but, when there have been RSD releases by Mr Bob in the past, I am pretty sure that they were only available at "B&M" record shops and not on Amazon.