Stereo broadcasting was rolled out on R3 from as early as
1962, Stanley, and O.B.s from that date were being recorded in stereo: there's at least one from that year preserved on the BBC legends label (Rozhdestvensky's DSCH4), so there should have been no obvious reason for poor quality in a 1968 broadcast, with six years of practice in hand. If the discs are sourced from a private collector's tape, then there are many more variables which might suggest why the sound is strange: bad reception, poor quality tape or a slow tape-speed, even tape degradation (over forty years, very possible).