What action can a real estate broker take if the client refuses to pay up?

We found the flat through the ex-tenant on Facebook. The ex-tenant recommended sharing our profile with 'A,' the owner's agent. 'A' had received brokerage from the ex-tenant when she rented the flat six months ago. The owner emphasized that 'A' would handle everything regarding the agreement. We assumed 'A' acted as the owner's caretaker cum agent. 'A' failed to coordinate a meeting with the owner, so we contacted the owner ourselves. We managed most aspects ourselves, with the agreement facilitated by external agents 'B,' who work with 'A,' and we've already paid them the agreement charges. The police verification had errors, which we had to correct ourselves. 'A' never mentioned brokerage during or after the signing, and she didn't assist in our flat search or owner meetings. Now, eight days after moving in, 'A' demands a full month's brokerage (42k) and threatens to issue a one-month notice. There's no verbal or written agreement regarding brokerage.