Rightmove tenancy check - query?

Hi all, I have applied for a new flat to rent in London, all checks went well except my previous landlord has disconnected their phone number and the real estate who I paid my rent to won't talk to rightmove, they keep telling them to contact the landlord. Rightmove were kind enough to just ask me for my contract and last 6 months rent. I sent that through but this is where there is a problem, in Sept my direct debit failed and I didnt realise. So I paid it in Oct and then next day paid Octobers rent so it was clear there was two payments in Oct, one for sept. I explained to rightmove it was an oversight but no one raised any delinquency or arrears claims, it didnt affect my credit report or anything. Now rightmove are trying to contact the landlord to ask if this was pre-agreed (as I explained it wasnt) otherwise its going to go on my reference as a late payment and will affect my reference check. Does anyone know how badly it can affect my reference check? Will it fail me? As mentioned, all other items passed fine such as affordability and employment etc.