I'd feel happier about the reasons for this zealous collection of anticipated dues if the PRS got serious about billing every radio station on the basis of a full submitted playlist. The 'sampling' method may have been appropriate way back when, but there's no excuse now, computer technology being what it is. Lost income from this source would for most songwriters far exceed that for random plays in sessions, as these plays stimulate proportional payouts from the pub/karaoke pool as things stand. At the moment the money goes in and, unless the writer self-certifies as Tom Bliss says, is absorbed into the general pool.
Until then, one could reasonably assume that there are other items on the agenda.