BT have never charged me a penny for my year of complaining. I am currently a happy BT customer, (but this could change) as since they put in adsl2+ in my local exchange, speed has been at 16 megawotnots.
However, prior to that I had endless troubles, and endless visits from BT engineers, over a number of months, as well as new modem/routers, and BT paid for new master socket.
I just decided I was not going to give up with the calls and insistence on Engineer visits.
I suspect you have to go through the umpteen things that they ask you to do on their "help" line first, to prove it is not your pc. (Might be worth checking that the settings are as they should be......all that "auto detect" stuff). Then do all the crackling line tests to prove it is not your internal wiring.
The call center people seemed to me to be given a script which requires them to keep saying it is your problem not theirs.
However, as mine was changing for zero to 4-6 megawotnots for no reason, eventually, after lots of phone calls and me insisting on "Escalation" during each and every call, whatever the issue was, they fixed it after about the sixth or seventh engineer visit.
I was lucky in that my modem router is only about 6 feet away from where the BT line enters the house, and although this house has lots of extensions, none are in use since we got the wireless phones that do not need extensions. .
As mixture points out, their "engineers" are of varying quality.