Was it released on cassette? I suppose it must have been, since most things were in the 1970s. I've got the original LP and, of course, it was re-released a few years ago as well as being released as a library album in 1980 and on CD in the 1990s. I would guess that a) Brian must still have the original tapes and b) could easily copy them or the LP or CD. So I'll speculate that, for some reason, he or Warren wants the commercial cassette release. I'll have a look on ebay as this is where things usually turn up!
P.S. WOW!!!!
https://www.ebay.co.uk/itm/BRIAN-BENNET ... SwKmVeZnSY