Aah now you're asking in the right place!
I work for Account Management with Vodafone BTW...
What you should do is this:
1) Check out deals with T-Mobile, Orange etc. online and in stores.
2) Phone 191, option 3, option 2.
3) See which deal they give you and if it's not that good then...
4) Say that you've seen XXXXXXXX with a much better deal and you think you might move to them.
5) They will then try to match it to keep you as a customer.
It generally works like that because there are almost as many mobile phones as people in the UK so they're concentrating on customer retention more these days.