Am I over proving my bread, or might there be some other issue? I'm using a fairly standard baker's mix. 750g flour, 450g water, salt, quick blend yeast (Doves).
I bulk prove for 40 minutes, then shape reasonably tightly and into a large, heavy duty loaf tin. 30 minutes in the oven at about 200 degrees c. (Fan oven).
You can see this effect in the image, where I've held it up against a light.