The way I complain with any telecom company is to follow their code of practice here:-
http://media.talktalk.co.uk/sites/Ta...f_Practice.pdf
I always write to the address, and I insist on them giving me a resolution date or deadlock letter in writing. (then I follow their escalation process... normally things are sorted before it reaches this point)
Also, your contract works both ways, and you can take them to court for breach of contract, as you will have to go elsewhere for your broadband and phone service and they are unable to supply what they are contracted to do. (installspark is well versed in taking this route, if he pops in I'm sure he'll give more info if you need it)