Yes, a light leak would make a light band rather than a dark one. I was thinking more like something loose in the camera that's getting in the way.
The second image with part of the next neg in it has the area between the frames scanned too dark to see if the band is there or not, but it doesn't appear to be in the next frame, or at least, not as distinctly.
May be completely irrelevant but this reminds me of a problem I had years ago with bands appearing on 120 film. The bands I was getting were across the frame though. To cut a very long story short, the film I was using was Tmax 400 (TMY) and the bands were caused by stressing of the film as it passed around the rollers of the film back. After much discussion with Kodak, they admitted this could happen. The effect was cumulative so the longer the film stayed in one position, the stronger the bands. Hence it only showed up when the film was left in the camera mid roll for a while. These bands that Nigel has look very much like what I got, except mine were going in a different direction and I can't think how the Yashica could stress the film where they appear.
Just for the record Nigel, what film is this?