Hi. We had fly strike just like this with one of our Isa Browns (not as bad as these pics - we caught it early).
We fixed it quite easily. I thought I would share our method.
Initially we thought the Chook was egg-bound. So we filled a plastic tub with warm (not hot) water (in between lukewarm and the temperature you would bath in, if that makes sense). We put her in the bath making sure her butt was completely covered.
After about 10-15 minutes we noticed what looked like seeds floating in the water. The seeds wriggled and we realised they were maggots. She started eating them. We kept her in the water for another 15 minutes (about half an hour in total) to make sure the maggots were off her. The maggots seem to need air, so if you keep her butt submerged for more than 10 minutes, they let go and look for air.
The next two days we did the same thing, except we added a good quantity of sea salt to the warm water. We kept her in it for 15-20 minutes. The salt water is great for healing. By the third day she was 100% recovered and about 10 days later she started laying again.