That's why I never bother with chicken wire. It won't contain anything except a plant, and sometimes that's iffy.
If you want to keep an animal in or out of an area, use welded wire. I'm not clear on how the 4x4s and bricks are placed around your pen, Marlingardener. Are they flat on the ground, so the chickens try to dig through them at the base of the fence?
To keep my raccoons, foxes, and other wild critters in their pens, I covered the bottoms with welded wire, then covered it with a few inches of soil, which I topped off with pine chips. They dug, but they couldn't get through the wire.
Of course they had learned to follow me around the yard when they were small babies, so when I let them out for exercise and foraging, I didn't have to worry about them wandering too far from me.
I have a neighbor down the street who has the same problem. I find their chickens in my yard -- front and back -- all the time. They haven't asked me for help, so I haven't offered any. I just shoo the chickens away. I wonder why the owner doesn't clip their wings? These birds can fly over my fences easily. I've even seen Daisy go up in the apple tree to chase them down.