Composers: the Muse departs?

[TerraEpon]

And yet (unlike Tippett) he is well served by the record companies, Chandos having recorded every last bar he composed

Er, not even close. His film scores were in concert arrangements or suites, and there's all sorts of small pieces missing. Peek through http://web.archive.org/web/20110525202655/http://www.williamwalton.net/ (sadly not online any more :-( ) and check out many which show unrecorded, or only recorded in someone else's orchestration/arrangement (or only a later version, etc)