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1960-70's horror/thriller - Man & Mistress plot to murder mans wife - set in an old boarding school

Remembered to be an old british film from the 60's-70's

remembered scenes:

- Man & (possible) mistress are in a room at the school, filled with furniture covered in white sheets, presumably discussing the murder of the mans (girl)friend/wife. The mistress is working on a clay bust/sculpture of someone & uses a knife to carve out the eyes of the bust

- in a pivotal chase scene, the man is running after his wife/gf  with a gun & thinks he's got her trapped in the room with the white sheets

(now there might have been a supernatural aspect to this, as I can vaguely remember the voice of someone who's supposed to be dead (old headmaster?) speaking to the man over a PA system)

The voice over the PA seems to be goading the gunman on while he searches for the woman. he thinks she's hiding under one of the sheets & shoots the sheet. blood pours out of the bullet hole, but when he removes the sheet, its revealed that it was actually the mistress tied up under there.

- At the very end, just before the credits, we see the woman has survived & has exited the school, looking visibly traumatized & begins to walk down the path away from the building. As she's walking away from the camera, it zooms out & pans up to a tree where we see the body of the man has been hung by the neck by a branch. (All the while a ghostly choir is heard singing from the PA system)
asked May 21, 2014 in Name That Movie by do-re-fus (2 points)
Diabolique
Thanks for the suggestion, but thats not it.
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