Put it in writing to Tesco with a copy of the order confirmation and threaten to involve trading standards if they don't sort it.

On 23 December 2011 I purchased a Samsung Galaxy S2 mobile phone for my wife on a T mobile 24 month contract. The advertsied deal was for £20.42/month including 300mins calls, 300 texts and a "Data Booster" package of 500MB per month. I received written confirmation of my Order stating the above deal (which I printed out) but no confirmation email of the order, only emails (from tesco) confirming I had passed a credit check and that the order had been shipped.
However, once I had received the phone and I had registered with T mobile on their website for billing, I noticed that T mobile were charging an extra £5.10/month for the "data booster" package. This was clearly advertised as included when I placed the order and I printed off a confirmation of this at the time.
I have since contacted customer services at both Tesco and T mobile but each compaany is blaming the other and does not accept responsibility for this error. In the meantime I am left paying 25% more per month that the deal I agreed to.
To make matters worse, Tesco are now advertising the mobile at the increased price I am now paying, albeit with an increased amount of included calls & texts. I can only assume that there was an error on the Tesco phone shop website when I was placing my order which has now been corrected.
I know I am not mistaken regarding the deal as I have the printed confirmation of what was included on the order I just feel that I should only be paying the amount that was advertised and the deal I agreed to.
Does anybody have any suggestions as to how I can take this further as I seem to have come up against a brick wall with both companies advising me to contact the other?