Wouldn't a light leak in the bag show up a little less defined? As in, not as sharp and clear, without any motion blur as it is in these photos? Somehow my limited knowledge of exposure fights me on agreeing that a split second of complete non-movement of your hands inside the tent would generate these markings. Unless the deterioration is so bad that it's pretty much a sieve, which you should have probably notice without any problems. And then the strange pattern around the sprocket holes also makes that hypothesis unlikely.

I have no better answer for you other than contacting Ilford. Maybe shooting Simon a PM here might get you a little closer to an answer.