If they want £4 per bill then write to them telling them you want a copy of all data held on you under the Data Protection Act. Enclose a cheque for £10 and they will return you all your statement for the past 6 years. You'll also get a copy of any other data they hold on you including recorded calls and CCTV they still hold on you (if you've ever visited their offices).
They have 40 days to comply if not then the Information commissioner will make them.
Once that's through write a detailed letter disputing the charges and tell them you want compensation for the period you've been without service.
In the mean time I'd recommend you look for alternative companies to do business with, after all who wants to stay with a company that would prove you owe them money.
HTH
Edit - I see they're sending out the bills for free (as they should) but if you have any further trouble then write that letter. You'll get further quicker if they know you know your rights