We recently moved house and the previous owners had been with BG. We wanted to switch from them and rang them the day we moved in to say so. They said fine, we'd be moved in 4 weeks. We'd have to pay them for the 4 weeks it took to move us. Fine.
4 weeks later they rang to say wouldn't we consider not moving? No. So, the moving process would start in 4 weeks--from then!! Rang to complain and was told it was fine, it was just their customer retention scheme people, and they'd say anything to keep customers. Er, right 
So today we finally got a bill from them for the short interim time they'd been supplying us--only it was a final notice, saying they were about to send round bailiffs!!
But it was the first bill we'd had from them! 
When I rang to ask wtf query it, they said "oh your final bill is [amount £40 higher than the letter said]".
No explanation as to why we received a threatening letter, no written bill for this higher amount. I paid it and took the operator's name and auth code for the payment, but AIBU to complain loudly??