Are you sure that's where they got it? It could be true, but they could have also acquired it by other means.
That strikes me as improbable. As I understand it, the reels of audiotape were in Neil Norman's possession for about 25 years, and he jealously guarded them. Getting him to agree to any kind of release was one of the biggest challenges that LLL had to deal with.
The public got access to the previously unreleased music via the LLL CD set. This is not
like workprint film trims that were mailed all over the country in the 1970s.