If it's CPW then you have to make a call, then send a letter, then call back because you get given the wrong address, then get suspicious and make three calls in the same day, only to be given three more addresses, point this out to someone and finally get given the real address, then get told outlandish lies and not be able to speak to anyone who can actually help because their PAC processing centre is in Portugal, then call Ofcom and come back armed with the information that they have to provide a PAC code within 72 hours of initial request and finally cancel your contract and get your PAC.
You can even try emailing their executive dept with your complaint and receive no response at all.
It's pretty simple really.
The you join someone else and realise you've been putting up with the worst customer service money can buy.