Daniel Bowen wrote:

I've often thought it amusing that so much paperwork is still kept in the
BBC archives about these shows, yet the shows themselves have been
lost/disposed of.

Yeah but from the perspective of the late 1960s/early 1970s the need for
paper trails was clear, the need to retain old programmes that were almost
certainly never going to be screened again and which were no longer sellable
was not.