Thanks for your help guys. I've only seen it on this one holder which is one reason I suspect it (and I can't see any light leaking in when the holder is in place with its darkslide out). I haven't developed the B neg yet so I don't know whether it's only this side.
My guess is that the light's entering from above the strip that holds the film down, somewhere around the hinge. The mark looks like a shadow cast by the end of the strip that holds the film to me. And it's on both sides.
There's a little bit of play in the hinged bar on the side where this neg was. So what I should probably do is replace the hinge tape and see whether that fixes it.