You mentioned you have Mycoplasma within your flock, did you have testing?
You should be able to find diy saline recipes online, you can use table salt.
Since you are in the U.K. contact your vet and see if they can give you meds to treat symptoms of Mycoplasma.
Amoxicillin is not effective against Mycoplasma, it has no cell wall.
You've got MG in your flock, so use whatever medication(s) you've used before to treat her. Common meds are...