Not being charged for broadband.

I have a billing problem with my broandband/telephone. Its in my favour but still worried.

I took out broandband/telephone with a big supplier 12months ago. It was £30 a month. There was no introductory rate. When I took it out, I requested an extra service on the phone. However they failed to add the extra to my account. I contacted them and they added the extra service to my account for 50p a month.

First bill comes and it is only for 50p. Second bill only 50p. So I contact them and point out the mistake and they apologise and say they will sort it out. Next bill comes 50p.

Anyway 12 months later I am still paying 50p a month for my broadband/telephone. I have contacted customer services twice. Both times told it would be sorted. Still 50p. I have even made an official complaint and told that they won't chase me for the money as it is their error and the problem is sorted. Yet I am still only being billed 50p a month.

I am tired of trying to convince them to take my money and short of putting £30 in an envelope and posting it to them, I am not sure what else I can do.

My question is, if they ever work out the error, what happens? It is their error and I have tried three times to get them to take the right amount. I am worried about being given a huge bill. I also worried about being cut off for non-payment.

If anyone has any ideas would be very grateful.