Sore chickens - can anyone help please?

I have 6 chickens, which started laying on January 1st :)
4 are, I think, Cuckoo Marans and they seem very healthy.
The other two are huge white beasts, which I think may be the local meat birds.
One had trouble a few days ago, looking egg bound with tail down and a rather purplish comb and wattles. I bathed her bottom with lukewarm water, and she is now much better, but both she and the other white one have very sore patches with no feathers and red skin on the lower parts of their breast, running almost to between their legs.

They have a large run during the day, with access to their night accommodation -a large henhouse with perch and thick straw on the ground.
The other occupant is a Muscovy drake.

I'd be really grateful if anyone can suggest what the problem is and how I can help them.