I have been doing some more reading. The wires are definitely PVC. We still have a bakelite fuse box. I suspect that the wiring in the loft wand to the switches as probably replaced in the early 1960s. Apparently this early PVC is not as long lived as the modern stuff.
It could be that a rewire was done in the 60s and the switch drop cables were not replaced as they were in slip conduit, wouldn't come out, and the owner refused to countenance damage to the decor.
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