My cunning plan of shooting some more last weekend came to naught (daughter suggested we venture down to Westonbirt instead). But I did take John
@thedarkshed up on his kind offer of a cuppa, some scanning and a chat, very enjoyable. We had a good look for light leaks, and the good news is the bellows is sound and no obvious leaks in the camera.
John's theory was that the hard edge to the light leak must be because the light was coming in between the DDS and the camera, with the hard line being a shadow cast by the edge of the DDS opening. I had inserted the DDS from the left hand side (RHS in the image). Fiddling around a bit, it soon became clear that the most likely explanation is that I moved the DDS slightly away from the camera body when removing or inserting the dark slide. We compared the Chroma with John's MPP, and latter has much stiffer springs holding the back in place. The good news is that it's an easy fix if you remember (and are disciplined): just hold the DDS against the camera body with fingers and thumb of one hand while removing the dark slide with the other. John realised that is what he'd been doing. Of course, if you insert/remove the dark slide under your dark cloth, that would also reduce any chance of leaks. I'm going with the jacket or T-shirt solution for now, so that might not work for me.
Steve, if your investigations confirm this, would stronger springs be a solution? Or some kind of lock to hold the DDS against the camera back?