This extra charge also happened to me after the upgrade from 750k to "Mb.
I simply phoned NTL billing dept and informed them that the upgrade was suppose to be free.
They confirmed this and they re-credited my account accordingly.
You should not have to pay in advanced.
Unless of course, you are a new customer and have only just join NTL services.
I seem to remember that I also paid in advance when I joined, at the time it was called Diamond Cable.