How discouraging! I am glad that the antibiotics seem to help her, but discouraged about her continued weight loss.
Whenever we don't have permanent improvement with "our best guess treatment", then it is time to get some diagnostics.
Here in the US we have a credit card available almost immediately at most veterinary hospitals. It's called Care Credit. You can get approved right away and then make monthly payments toward your bill. Generally, the first 6 months is interest free!
Perhaps your vet there has this also, because I do encourage you to move forward with Tinkerbell's diagnosis.
One other problem I see are the repeated "jabs" (I love that you British use this term!) of steroids. This long acting steroid has a high incidence of causing diabetes
. Perhaps Tinkerbell is dealing with this now too. A blood profile as well as a urinalysis will tell us, as well as check all of her organ functions (except for the GI tract).
So, I do encourage you to have her blood and urine tested. I really do hear your saying that you don't have much money, but we have come to the point with her that we need to diagnose her specific problem so we know what to do for her.
Also want to tell you that not eating and weight loss is a sign that something is not right! Cats don't choose to not eat.
I hope that all works out for Tinderbell! Please reply with further concerns and questions you may have.