Plusnet can't get my bill right

[#1]

I called up to cancel my Plusnet service as I was planning to move to a cheaper deal.
The agent matched the deal which was essentially £12/month for broadband and phone plus a small charge for caller id.
Three months on now and I am still being charged around £18/month. Each time I call up I get told that the previous agent set it up wrong and that they will fix the problem and to not worry. Unfortunately this is repeatedly happening and I am monthly spending around 30-45 minutes on hold waiting for an imaginary fix. I continue to be out of pocket!
What is the best way to get Plusnet to honour what they promised? I can't be spending this much time on each phone call.
Any suggestions welcome.
(Current wait times 30 minutes).

[#2]

best suggestion is to wait for it to be picked up on here - even if it may not be until next week

[#3]

Finally got through on chat (much easier than customer services plus I get a copy of the conversation).
All sorted.