Phat Beamer: Therefore it should be okay to say I will not pay up.
Indeed, That's what I alluded to in my first post in this thread and in other posts.
If the circumstances are right the Code applies. The issue is whether BG agree the Code applies in this particular case or are going to attempt to wield a big stick as per normal. It's whether the latest demand is for the previous bill which they are asserting you didn't pay and you can't prove you did, or a new bill. The Code applies differently to those two scenarios.
Yep, put your case to them and see how they respond.
One thought that has crossed my mind, you paid but due to an account transcription issue (at the Post Office perhaps) the money didn't reach your account.