problem with O2 bill

msuhhail

Hi All,
I had contract with O2 2 years ago, I got free minutes/texts messages as well free Internet access for a year. last month of my contract I received 80£ bill when I enquired about this customer services said this is internet usage bill, its supposed to be free last month as well. But later they said its system fault.
I asked them regenerate a new bill and post me but got answer NO, I should pay this amount and they give me back next month, I cancelled my DD. So sent me few letters regarding 80 £ but I was asking for my 30 £ bill.
I sent 9 letters to O2 in final letter I wrote them what I singed agreement, according to my agreement if O2 change my tariff then they should send me written notification at least 28 days before changes and I have right to cancel my contract.
After that I heard nothing from O2 but I got check on my credit history. Now I am like defaulter from O2. I applied for mortgage and loan all declined.
I am sure I should take them in court now I have all proof of paperworks. If this O2 system fault then what my fault?
Please guide me how I can take a legal action against them , I am fedup with all this now.

Thank you in advance for guidelines.
Soha