@LydiaT In my experience the vets who actually know how to treat chickens are very few and far between, so don't let not seeing one if you don't have the resources bother you too much. The advice you get here is just as good as what most vets would do for you. Remember antibiotics take a while to work, so continue giving them. Can you get someone to help you hold the chicken's head still while you lance the boil? You really must get the gunk out of there. It will be thick, smelly and rather disgusting when it comes out. have one person hold her head, and one person quickly prick the boil with either a small sharp scalpel from the chemist's, or with a very sharp long sewing needle that you have held over a flame to sterilise it first. After lancing the boil you need to squeeze the gunk out of there. Put some rubbing alcohol or hydrogen peroxide3% on a kleenex and press/squeeze the gunk out of the hole you've just made. She will struggle, but it's not hurting her too much - just like squeezing spots. The pus will be thick like cottage cheese. Don't be shocked. This is what chicken pus looks like. After getting out as much as as possible (all, hopefully) topically apply a teaspoon of coconut oil with 4 drops of either tea tree oil OR oregano oil diluted in it. rub this on the boil.
Is she still eating and drinking?
If you are not fermenting her feed, add 4 drops of oregano oil and a half teaspoon ground black pepper to her ration of food that has been mixed with water to make a porridge and mix it in thoroughly. If you ARE fermenting her feed, just add 4 drops of oregano oil plus the ground black pepper to her feed and mix thoroughly. Give her this every day for at least 6 days, longer if you don't see improvement.
The oregano oil needs to be the best quality you can afford - 100% pure and containing 86%Carvacrol oil. It looks expensive for a pot but there are 20 drops in 1 ml, so a glass of 50ml will last you a long long time. Oregano oil is anti bacterial, anti viral and anti fungal. You can use it in future for treating yourself as well as your chickens and other pets if you get sick. If there isn't a dropper/pipette supplied with the oregano oil you can get one quite cheaply at a chemist.
Also add a soluble vitamin mix either to her water or to her fermented/wet feed. She needs to boost her immune system as much as possible.
You can give her these supplements either as stand alone medicine, or as a supplement to the antibiotics you are giving her.