The Tolkien Society produced a transcript of the entire recording (available for purchase at the time, and later reproduced and available from Beyond Bree in the USA as well), so it has been heard by a few people outside of the BBC staff at least! I would not be surprised if there is a recording in the TS archives and/or the BBC archives as well as with the Estate. I am pretty certain it has never been commercially available.

There was also another source...

BH