Change of contract

Good afternnoon
I ask for steer in regards changing the name in my contract.
I have since 2014 a contract with O2 which was made in the made of my ex-husband. It happens that i got divorced and since then I've kept the contract as i am paying the bills and just updated the address following changing house (as that seems giving less work having in mind that is very difficult exchanging by phone in English with O2; and i've all of the passwords).
Recently i was receiving some extra-bills to pay (bills received by sms) and asked a friend to update the mobile plan. It happens that some confusion happened at that time with the O2 home plan and now i'm receiving every month ad-hoc bills.
In order to solve once-and-for-all this confusion, i would welcome help to know what i should do to change the name in the contract (my name instead of my ex-husband). I add that it is impossible that he does anything to cancel it himself as he lives now in Saudi Arabia and i have no kind of communication.