After years of searching I finally caught up with GWAED AR Y SWER yesterday. This Welsh-language black comedy horror from 1975 has apparently been screened by S4C on tv, and also played at the National Eisteddfod and at a 2011 event staged by Twisted Nerve record label boss Andy Votel and Super Furry Animal Gruff Rhys (who remembers seeing the film as a kid in 1976). Standard body-count fare, running for 57 minutes, fairly easy to follow without English subtitles. The wacky premise has a sinister-looking choirmaster and his young charges bumping off a number of real-life Welsh celebs, the big local names (including rugby ace Barry John) having been booked to appear at an event at their school. Fearing that these appearances will take the shine off their own musical contribution, they set about doing away with the stars. Death by snake, electrified harp, explosive football, etc, ensue.
Sounds like an interesting oddity. File with 'Deafula' and that Esperanto one with Bill Shatner (which I have somewhere).