..and the results are in.
Turns out I was right from day one, she is indeed a female.
I actually had lots of people who told me that she was a male (not necessarily on this forum) and this never shook my belief of her being, well, "her". But when I went to the vet for a check up last week, I couldn't resist finding it out once and for all.
This means I know the sex of two of my birds (Red-bellied parrots being sexually dimorphic), there is three more to go...