Hi Joe. You signed a contract agreeing for them to take your money, so you can’t really complain if they do. As far as I’m aware, the fact that they haven’t taken any payments from you yet doesn’t change the legal standing of your contract (but I’m not a lawyer, so I suggest getting professional legal advice if you want to take any action).
The finance company will notice its error eventually and could well try to take all the backdated payments in one go. You’d hope they would call you to discuss this with you beforehand, but they may not.
Best suggestion would be to take the initiative and call them to agree a schedule. That way, you can make sure that they don’t suddenly empty your bank account in one go.