Nothing much more to add except that Vodafone are in breach of their own terms and conditions. There was no network issues that caused the problem as you could make calls. It was purely down to their extremely poor customer service.
In my opinion, due to VFs breach, you are not bound by the terms and conditions even if they added the line " If one term is deemed unlawful, the remaining terms will continue" (or something like that.
It is my belief that Consumer Protection Regulations will protect you as VF are in breach and these new letters could amount to harassment which is now a criminal act. I was only reading a guide earlier today which you may find interesting.
Follow the link and look at the guides.
Please calculate how long you have had to spend on this issue then multiply it by your hourly working rate then add on any call charges you have incurred and any postal/stationery costs then present VF with the bill.
As well as clearing this account, removing all traces from your credit file and offering one of their oh so famous 'Goodwill Gestures' you want compensating for the time and effort you have been forced to do.