I found P4U were able to offer better deals and had more flexibility than CW. Just keep pushing them politely and see what you can get away with.
P4U started out with a £41pm for a 24 month contract, then offered an automatic £7pm rebate and then threw in £60 cash back. That was for 900 mins, 3000 texts and 1gb of data and a free phone. Its worth knowing what upgrade banding you are.
I got mine over a month ago so you should be able to get a better deal. £20pm might be pushing it though.