Phones 4 U-bad service or what?need help

^opm^

in brief-i took out a 12 month contract thro phones 4 u shop on a phone-after 6 months u sent of ur 1st and 6th month phone bill to claim back ur first 3 months line rental,well i sent mine of last month with my receipt etc and they didn't receive it-when i told em where i sent it to they said i sent it wrong address-anyway they wont give me a new claims back form to get my money and say there is nothing they can do
anyone any idea's how i should go on-im fuming not over the money but the principle
whats legal position i thought sod em i will just cancel my direct debit to them 3 months early and stop them getting anymore money but if i do this they would just cut simcard off and probably put me on credit black list so any idea's? please