Hi there,
I'm pretty sure one of the previous responses to this was to find out the complaints procedure and follow it through.
Also consider finding out who the chief exec is and writing the complaint direct to him.
You should be pushing for them to honour the stated price plan for the whole term.
Alternatively you may be able to get them to reduce the plan cost.
You could have a look at what tariffs are available from Vodafone that are comparable for new customers, and think that a loyalty of £5 a month difference would be an acceptable price.
Have a search of the forums to see what advice / contact details have been given to sort them out.
As always try to remain clam, assertive and polite.
.
If the contract does not exist, how can you be charged for an indeterminate service plan (tariff)?
As it is a credit agreement / contract, you may be able to request this under the CCA.
If there is no contract, then what terms are in place to keep you locked into your phone? You should be able to leave them and go to someone else.
Good Luck
Sparky.