Unfortunately there usually is no "cure" for Egg Yolk Peritonitis.
Antibiotic treatment may assist in supportive care/relief and make your hen more comfortable.
Duramycin 10 is usually used to treat respiratory illnesses. Not to say it won't work, it could help fight some infections due to the Peritonitis.
Quick search came up with one reference I have found with a "cure" -- article linked below.
I encourage you to research the different antibiotics available and what illnesses/diseases they are typically used to treat. There are numerous articles/threads here on BYC and the web addressing Peritonitis as well.
Drug Info: https://www.drugs.com/vet/duramycin-10.html
""Control of chronic respiratory disease (CRD air sac disease) caused by Mycoplasma gallisepticum and Escherichia coli, infectious synovitis caused by Mycoplasma synoviae susceptible to tetracycline hydrochloride.""
Hi, sorry I completely forgot I posted this! Thanks a lot for the reply, this is very very helpful. I ended up using something called Lincospectin which seemed to help for a while, but I shall have a look at those links.