Well they called me up. They were very proactive and friendly in making sure I understood my invoices and why I started out paying one month in advance but finished paying one month behind.
The explanation comes out something like this;
I started out paying a month in advance five years ago. At some point (presumably when I payed £15 to become adsl max) my line was regraded and my billing period changed but not my billing date, I should have recieved a credit for the payment I had already made for that month, and the days I paid for and my billing date came askew, out of sync slightly.
The fact my bills became exactly one month askew is apparently a rare and unfortunate coincidence. It could have been just a few days askew, mine just happened to be exactly one month....Thus they say putting "in advance" on the bill makes sense for most customers, its just a random accident that my bills ended up exactly the opposite.
So there we go. They were certainly proactive and friendly about assuaging my confusion.
If there is a moral I guess it is read your bills all the way though, what starts in "advance payment from nov 2nd" may well end in "to october the 2nd". That and life is a confusing place. After five years it does not make a huge difference either way so onwards.