You should clearly put down in a letter to Orange what has happened. If you don't, they will be able to say in a couple of months that they have no record of the conversations. If you've not paid then you are in a stronger position but you should expect them to get awkward. It's really up to you to decide if you are willing to stand up to them, in court if necessary, or not.
Legally speaking, if you were told something specific like this on the phone, then that is binding. If you record the details in a letter or two and Orange can't produce a contradictory recording or transcript, then a court is likely to see that you are in the right. As I said though, it's possible that Orange will want to get awkward.