Just to try and clarify;

This copyright is only on the public performance of a commercially issued recorded work.

When a recording is made and issued, that actual piece of work will earn royalties for all the musicians that partook in the recording, featured performer, accompaninsts and backing musicians too,whenever
that sound recording is played in public. For instance a radio broadcast, background in a restauarant, shopping centre, lift, ANY public use of that recording. This applies for 50 years at present.
It does NOT apply to the writers of the lyrics or music, these are covered by a different set of copyright laws.
If another take was made at the session and was itself issued at a later date, then that itself is a seperate piece and will have the same rules applied for fifty years after it's issue. Also if the record company takes the original recording and does a remix and issues that it is counted as a new recording and starts another fifty years.
With material such as folk music or related minority interest music, the amount of public exposure is so small that it often falls below the radar of the people responsible for returns to the royalty collection company.
It would appear that PPL are going to appeal on behalf of the musicians. Personally I feel that it is really the record companies that are the real beneficiaries. You see, they get fifty percent of the royalties to which the above refers. Plus I believe it is planned that if performers do not register to collect their PPL royalties (which costs the performer nothing) then after a while their unclaimed monies will revert to the record companies.

This is as I understand the situation was up until a couple of years back.

I would recommend that you contact PPL direct if you have any queries, they have a performer help desk and have a website telling all about themselves. They will also be able to confirm or otherwise if my information is correct.