I'll be seeing Doc next month so I'll ask him, but other than that The Vaughan Williams Memorial Library might be a good place to start. Try the EFDSS website. I don't know if Cyril was registered with PRS.

My copy of the book states that Grey Funnel Line is copyright to Gwyneth Music Ltd so it might be worth Googling them. Off the top of my head the people who record his songs nowadays just rely on PRS picking it up sending royalties, but to whom I don't know, possibly Sir Cliff and Sir Paul if rumours are correct.