BastyPutt
Yes, your Polish is a cockerel...
Also this. The abstract is great, and also from a government source.Your chickens likely have a chronic respiratory disease (CRD) caused by Mycoplasma galisepticum. (MG) Chickens pick this up and it takes up residence in their cells and then when they experience stress, it flares up. Often, it resolves on its own and natural remedies are fine to use.
However, if it lingers and even becomes worse, and it sounds like it has, then an antibiotic may be needed or some of these birds could die. Often what happens is bacteria moves in to infect the mucous and inflamed tissues. This is what the antibiotic is for.
@nuthatched has given good advice. Tylan or Tylosin are the best antibiotics for this issue. You can get Tylosin here without a prescription for now. Next month that will change as the federal government has made it mandatory these antibiotics are to require a prescription to buy. So get it now while you can. https://homelabvet.com/product/tyclosin-500-30g/