Yes, the informant must be on the GRO cert.
The reason I suggested trying the local RO as well is because every time something is copied, it increases the possibility of an item being misspelt or omitted.

I think you need to start at the beginning again, and find everything possible about the baby's birth before moving on to her mother. If your aunt has the original birth cert will she scan it to send to you, or ask someone to do it for her? Much better than depending on what she says or remembers. And also ask your mother to scan and send the GRO cert? Or if they can't do that, to photocopy and post to you. The certs should be the same, but look carefully at everything written there.

Sue