You say:
"As CM does not (at last time of checking) hold an MCPS licence for CD pressing (indeed they no longer have a pressing plant), it is rather difficult to say they are doing anything actually illegal. Borderline, yes."

Not a criminal offence but a civil one, I think, and the perpetrator can be sued.

If there is anything on the CD(R) to which MCPS (or their members)have rights, and there is no MCPS logo, then there is clear evidence of wrongdoing and they can take legal action to recover the amount due.

If we look at all the CDR issues, for all artists, then we might well find something to act upon. Song and arrangement copyrights are notoriously difficult to research, including folk songs. Look at Paul Simon and Scarborough Fair, as one example.

PRS are for PERFORMING rights, MCPS for PUBLISHING as I understand, so the PRS would not be interested here.