George,
The Library of Congress was the source of the Grainger cylinders, mainly of Joseph Taylor, which were used for the "Unto Brigg Fair" record. The recordings were remastered by Bill Leader, I don't know if the L of C got a "safety" copy or not. As you say the cylinders are now 90+ years old, so your guess is as good as mind as to what could be retreived at this date.
Dick Gaughan, while still working with Bulmer remastered some of the Leader archive "Fate o' Charlie", and "Ballads and Songs" were two he mentioned by name.
I understand that "Kist o'Gold", has not been released on CD because the masters have deteriorated to such a degree that they are unusable. Remastering from mint vinyl, would seem the only option. Dick's words were "I'd just like to see it out there, I won't see a penny".

For anybody who doesn't know tapes, being magnetic, suffer from what is known as "print through" where some of the recording is passed from a peice of tape, to the tape over and under it in the spool. This results in a "echo" of the next bit of recording. It is most often heard at the start if the recording or on unacommpanied singing. Some of the Leader records had this detectable when the masters were new, so what it will be like after 30+ years untouched, who knows?. This is in addition to physical decay.

Pavane, Peter Bellamy jumped about between recording companies, Transanlantic, Topic, Free Reed, spring to mind. The only track I know for sure on Trailer was a Young Tradition song on the Trailer Sampler, but there may well have been some, ?the Kipling records for instance I'm not sure.

Tyke, Mr Bulmer is not above making copies himself. I know of one group who found that copies of their self produced, vinyl only, album started to appear on cassette after Celtic Music started distributing it.

love, john